As someone who probably should have been raised with ASL and Deaf culture, but now at 38 is just 'getting into it' in the last few years, and having to teach myself everything and having no access to Deaf culture, I have mixed views. I think if anyone wants to learn ASL they should. And in fact I wish more people would because I actually rely on the language to communicate, because I cannot lip read and I cannot hear well enough to follow a conversation (even with hearing aids), and one of my biggest struggles is that nobody I know - friends, family or otherwise - can sign and nobody wants to. And even if they did, they expect me to teach them when I don't even know how to teach myself. So any hearing people who want to learn, awesome! Do it! Because on a personal note, I need more people who can sign, so that I can communicate with more people, and not feel like I'm in a box talking to myself all the time.
However, something to be aware of is that in some areas, hearing people have more access to ASL resources than d/Deaf/hard of hearing people. This is the case in BC where I live. There are literally an handful of places in the province that offer ASL training of any sort, whether it's an actual program or individual classes. There is nothing on the community level, except for one that I've heard of (there's probably more but the point is, the help is just not there for most who really need it). Even the training programs they do have are all catered to hearing people: they're interpreter programs, they cost thousands of dollars, and they're meant to advance careers and give people more job opportunities or more skills in jobs like, say, working in the community or non-profit stuff. They give people an 'overview' of what it's like to work with people who are d/Deaf/hard of hearing. That's not to say that d/Deaf/hard of hearing people can't take them; of course they can. Anyone with the time and money can. But as someone who has taken even community classes, I can say that in a room of about 20 people, there was maybe one other person there who was there because they were d/Deaf/hard of hearing. Everyone else was hearing, learning for a friend, learning for a job, a family member, etc.
Here in BC, there are plenty of free resources for hearing parents to learn ASL to communicate with their hearing babies because they've figured out that sign language enforces bonds between parents and child and because it improves infant development significantly. Some resources also exist for hearing families with deaf/hard of hearing children. But there is nothing for individuals of any age who want to learn it for themselves, unless they are willing to pay 100s or 1000s of dollars for it and learn with other hearing people, deprived of their own culture. So while I'm not implying that every single d/Deaf/hard of hearing person who responds less than favourably to hearing people learning how to sign is doing so because of these things, it's helpful to realize that being d/Deaf/hard of hearing does not automatically mean access to sign language, nor does it mean access to Deaf culture, and many hearing people make that assumption. There simply isn't much awareness about the reality of what it's like to access ASL/other sign languages/Deaf culture if it's not something that you're born into. And also, many children who grow up d/Deaf/hard of hearing never have family members or friends who try to learn anything for them. They're sent to hearing schools and forced to 'make it' in hearing society because however many years ago, a bunch of hearing people decided that was the best way to 'deal with' deafness, and that Deaf culture and sign language needed to be deprioritized for that reason.
It's just a matter of educating yourself. Like with any other community you're not a part of (whether it's based on ability, language, ethnicity, age, gender, sexual orientation, etc). Always assume that you don't know the entire picture. It's great you're learning sign for someone else. And it makes me sad that you're not getting more support for it. From my limited knowledge of Deaf culture, anyone, hearing or otherwise, who is making an attempt to sign is welcomed. But d/Deaf/hard of hearing people are not homologous; we do not all feel and think the same way about everything. So people will react differently to different things. Ultimately I wouldn't let it get in the way of you learning for your brother, or signing with other d/Deaf/hard of hearing, or even hearing people who can also sign, and protect your boundaries when someone's treating you unfairly, regardless of the reason. But I also think it's important to be mindful of difference, while celebrating similarities.