I honestly think your lack of caring says a lot about you. I would doubt you TRULY love this b.f of your or else you would be more easily able to connect with how he feels about his brother and his special situation. These people in here seem to like to make everybody feel better about having messed up thoughts.

I have a male friend with a situation like yours and he does anything and everything to be there for his g.f and her family. If you already find yourself thinking and feeling like you do after only 2 years then I see a big problem for you in the future. I don't know how old you are but 2 years is NOTHING in terms of a meaningful serious relationship. The way you were talking I would of assumed that you either never loved this person as much as you think you do or you've been in the situation for 10 years plus. You should do your b.d a favor and let him go so maybe some day he can find someone who will love him enough to have a certain level of maturity instead on only worrying about how things will be for YOU. I find most people would sing a very different tune if it was their own family member.