How should I go about asking my brother if my boyfriend can move in? ?

I don't really think this is a good idea. Your brother and his wife have made a huge allowance by permitting you to stay, it's very difficult to have another adult living with a couple. They must care deeply about your welfare for them to have you there. I would be inclined to save as quickly as possible, as well as your boyfriend, and get your own apartment. If you have to go without or sell stuff off to do this then do it. I really do think even asking them could make them wary. It's good that you have stopped and wondered at the appropriateness of it because a lot of other people wouldn't have the same regard as you have. Good luck with your boyfriend and your future with him.