Forgive me, but this situation does not seem overly complicated to me? I agree with most in here on two points:
1. It's none of your business (but read on)
2. Yes they should be paying more
I'm sure if your brother actually thought about it for a moment, and he probably has done, he'd realise that he's not pulling his weight within the family, and if he has any stones at all, that'll be gnawing at him. That said, everybody is different, he might not give two hoots about anybody else's situation, and might just want to make sure he gets as much money as he can.
Your Mum on the other hand, obviously wants you all around. This makes sense, any decent Mother often finds it difficult to comprehend their child leaving, especially if said child is young and even more so if they don't genuinely believe that said child would survive well by themselves financially (as you have indicated). If however your Mum needs the money, and she's really struggling currently, that makes this situation a completely different one (though it still makes it up to her what she wants to do).
You only really have three options, talk to your brother, talk to your Mum, or do nothing.
If you talk to your Brother, do it properly. If my elder Brother had come to me a few years back, and said "You need to pay more", I might have thought about it, but I'd probably just have thrown it aside and wondered why he thought he could tell me what to do. If however he'd come to me and said, "Listen, man to man, I think Mum needs help money-wise, and while I know you don't exactly have buckets to spare, I don't think you realise how much difference what little you do have to spare could make", then straight away I'd be thinking how horrible I'd been to take advantage of such a petty rental price in the first place, and would have changed things.
If you talk to your Mum, do it without your Brother present (obviously), and make sure you find out what she actually wants. At the end of the day, if she thinks a lower rent will keep them around longer (and she wants that), or that a lower rent will help them save more to move out quicker (and she wants that), then clearly she knows what she's doing. But you won't find that out unless you have a serious conversation, not a quick one over the counter where she tells you the bright side of the story without the truth.
Personally, I'd talk to your Mum first and see what she wants, if she wants him to pay more, and she's definitely not going to do anything about it, then talk to your Brother, but do it like his Brother, not his Dad.
