How to help my (hopelessly underweight) brother see noticeable benefits of weight-lifting in 6 months?

Your brother is doing something that falls into your area of expertise and it's expected that you're excited and drawing up scenarios. Although I feel it's worth respecting the bar your brother has set himself, no matter how low it is even if he is a beginner and may not know any better. To be honest, his idea is pretty smart if it's how you described - work out at home casually so that he could be interrupted to help out his family.

You mention that he has been skinny his entire life, but when you asked him about building muscle you say it looked like it caught him off guard. My conclusion is it's possible it doesn't bother him that much and he may never actually lean more into bodybuilding as you're hoping. Having a more ambitious goal would for sure make him do that and as a side effect allow you to become a proper mentor, but it sounds like he just want to work out unto itself and he's ok with his body. He's healthy weight after all. You'll see in time if his motivation wanes in time and you could step in, anyway.

I'd say treat it more like a cool thing your brother is doing rather than a project where there's your responsibility in the mix.

In your OP you only mentioned that your brother just recently got back into weightlifting but there isn’t anything about his goals in relation to that. Is he just trying to get his feet wet to get into the habit of exercising just for the sake of it (to “stay in shape”) or did he outright tell you that he wants to build muscle (and you didn’t just assume that he does)?

I talked to him today and it seemed to catch him off guard. He basically just said it's a good habit to get into and he considers it exercise more than bodybuilding. He has 3 young kids under 5 so I'm guessing that his main reason is to do exercise in a productive way that lets him still help out around the house like if a kid starts crying or breaks something, he can immediately stop what he's doing and take care of it.

I agree with the person you’re replying to that a hands off approach would be best. Honestly the routine he’s doing is fine when it comes to just getting general movement in. I think the most important thing is getting him to see exercise as “something that you just do” like brushing your teeth.

Yes, that would be great but I fear without a goal to focus on, he will probably quit before it becomes a long-term habit. As you probably know, when you involve a habit with a long-term goal, it not only makes the habit more meaningful, but you'll also dump 100x more effort into it rather than (like brushing your teeth) just "going through the motions" to get it over with. I'm also low-key hopeful that muscle-building will increase his appetite and help him put on 30 pounds of lean mass. He's been toothpick-shaped his entire life and I can't help but see his body type as sickly (even though he's very healthy and much healthier than me).

Where did you get that from?

From the title:

(hopelessly underweight) brother

But:

Even from the screenshot,

Did you really just ask why I didn't travel to the future to click a link that hadn't been posted in reply to my comment yet before making the comment?

I like Paradise too bro, but real life ain't TV

In your OP you only mentioned that your brother just recently got back into weightlifting but there isn't anything about his goals in relation to that. Is he just trying to get his feet wet to get into the habit of exercising just for the sake of it (to "stay in shape" or did he outright tell you that he wants to build muscle (and you didn't just assume that he does)?

I agree with the person you're replying to that a hands off approach would be best. Honestly the routine he's doing is fine when it comes to just getting general movement in. I think the most important thing is getting him to see exercise as "something that you just do" like brushing your teeth. This is achieved more by e.g.:
- joining him on his workout and having it be 2.5 hours long instead of the usual 1 hour because you're running your mouths so much (social aspect)
- having archival Rich Piana videos running in the backgrounds for the meme factor

rather than sending him a Google Sheets spreadsheet for the Jacked & Tan 2.0 routine for the ultimate minmaxed muscle growth. "Physical Education" in school is meant to "Educate" you to appreciate and incorporate any sort of movement into your life even after graduating. Sadly it just boils down to playing some ball games. So hopefully he can put some physical education into himself this time around.

Becoming a gymbro is very much a brainwashing process. Appreciating a "sick pump" isn't really immediate, considering good programming & browsing nih.gov for studies isn't either. When I started I was going to my middle school gym after hours because a P.E teacher was staying after hours so we could be supervised and be allowed to use the gym. I was doing the most dogshit workout imaginable, but at least I got "Physically Educated".

My serious gym days are behind me but I still go to join my friend during his workout, to stretch or do 30 minutes of cardio. I know I won't build any muscle due to low sets/week and no surplus. But I go because of the habit - exercising is something that you just do.

You may want to encourage him to buy a piece of workout equipment like a weight rack or bicycle and to start taking protein shakes, and just say that if it's not getting easier he probably needs to eat more. Exercise while eating more often makes you want to keep eating more

eating disorder

Where did you get that from? 145 lbs at 6" is 19.7 BMI, which is "healthy weight". 18 is the threshold for underweight. Sure it's skinny but jumping to "eating disorder" isn't exactly welcoming to skinny people struggling to build muscle.

Even from the screenshot, he just looks fine.

Or invite him for dinner and cook something nice. If anything will work here, it's positive feedback.

Oh I forgot icecream came about through evolution, silly me, what was I thinking ..

You get him professional help for his eating disorder…

I'm 8 years older than him and he's always been very underweight his whole life since as long as I can remember. Here is a timestamped youtube screenshot of how my brother approximately looks (very tall and skinny but not sufficient to suspect eating disorder). If it were an eating disorder, then at some point in life he would have been a normal healthy weight but he's always been shaped like a toothpick. 😣

Like, you need to get him to eat, not exercise.

How exactly? Should I just frame it in a clever way so he thinks of it as "non-meal eating" or similar? (i.e. if I tell him to eat 500-600 calories before and after his workout, with at least 40g of protein each) I also thought about framing it as FOMO (fear of missing out) and that he's leaving gains on the table if he isn't in a daily 200 calorie surplus, which makes it sound not that difficult to eat an extra 200 calories per day.

In metric: he is around 1.82m and 65 kilos which puts him at a bmi of 19.7, which is at the lower end of a healthy range.

Here is a timestamped youtube screenshot of how my brother approximately looks (very tall and skinny but not sufficient to suspect eating disorder) since I should have provided an image in my OP rather than give his height/weight.

As a beginner doing anything is better than doing nothing, so why not just be supportive and offer to help/explain stuff but don’t give unprompted advice. There is so much you can already achieve with some weights at home and he doesn’t have to go full gymbro to see meaningful changes for health and fitness

I agree and I hope to just give him encouragement and let him know that I'm very knowledgeable and will happily share info with him about any topic as it relates to hypertrophy or muscle protein synthesis. However, I'm not sure if I should get his hopes up that he will gain any muscle or strength, given his lack of insight into eating more everyday. He might be self-conscious about his thinness and I don't want to make him feel bad about being underweight even though that's the #1 thing that will hold him back from seeing any benefits such as strength or gaining muscle.

Op:

My brother won't eat

You:

He should eat protein! He can't just drink something like fatty cow milk, nothing on earth has ever grown and gained muscle mass from fatty cow milk! How could anything ever gain weight and muscle mass from something evolutionary designed to cause an organism to gain weight and muscle mass?

Like bro, you could have at least asked me questions and learned something.

Now you have to find someone else.

Like I said "at least 2500", he's pretty light, his maintenance calories will be about that. He can build muscle at that if he's untrained. He needs to focus on protein if he wants to build muscle at mid 30s, if he was 20 then sure, icecream milk shakes or whatever.

If he's 6 foot 145lbs, he needs to be microwaving ice cream and drinking it...

It doesn't matter what the Macro is, he needs a shit ton of calories.

at least 2500 per day (or more) if he wants to gain weight

2500 daily calories is at the low end for sustained weight at that age group.

He can do all the exercise in the world, you can't make muscle out of oxygen.

would see reasonable gains

He literally won't because of conservation of energy...

Like, you can't just ignore physics.

In metric: he is around 1.82m and 65 kilos which puts him at a bmi of 19.7, which is at the lower end of a healthy range.

As a beginner doing anything is better than doing nothing, so why not just be supportive and offer to help/explain stuff but don't give unprompted advice. There is so much you can already achieve with some weights at home and he doesn't have to go full gymbro to see meaningful changes for health and fitness

As for the diet, maybe send some nice recipes, but don't push it either.