How to handle significant other’s family?

First I agree it's wise of you to address these concerns, you are at the beginning of your adult life and checking in with yourself, being aware of your needs are all important lessons not everyone learns about relationships so early. I have to ask, have you had any of these discussions with your BF? I mean he obviously isn't going to abandon his family, that's not who he is but given that's been clear from the get go it must be part of what attracts you to him but has he told you he expects you to financially support his family? Has he said he will always "choose" his current family over one he might start with you or that you will always come 4th in line of importance to him or are you surmising this? Has he said he wont accept and care for your family if and when needed? Do you know that his dream plan is to move you in with him and his 3 family members with the expectation that you will help him support the household (them) financially? Maybe it is something he has thought about and sees that way but maybe it isn't and you are getting way ahead of yourself and him. It might be that he has ideas/plans for other arrangements or it may be that he simply hasn't given it much thought. It doesn't sound like you have gotten in deep as part of the "care team" and that's a good thing but it would be good for each of you as well as your relationship to talk about what the future could look like, how can each of the needs be met for supported family members if the two of you decide to live together and then on down the line as family members needs increase? It isn't outrageous or selfish to want to start your lives together, living together just the two of you and he may very well want the same thing. It does sound like there should be help available and avenues for each of them (mom, sister, ex) alone or together to be on their own, it just might take some effort (maybe something mom could be doing?) but maybe addressing all of that has been just too much while he's in law school and leaving things status quo all living in the same house while he's focusing on school has simply been the easiest way for him to keep school the highest priority.

Try to remember as you approach these conversations with you BF that his commitment to his family in need is a part of who he is and was before you came into the picture so it has to be part of the man you love. Also keep in mind that you can love him without being in love with him and that's ok but if as you say you are in love with this man and want to build a life with him you will need to welcome the whole package, family and all just as he needs to accept your whole package and as long as your doing that it's not "his family" and your family it's our family and the two of you need to be making decisions together about the life balance of responsibilities you each bring to the relationship and you need to be willing to share those responsibilities (meaning you each make concessions as well as pitch in) but try to do it from a place of love rather than fear, watch out for each other rather than only yourself. Your BF may need help protecting himself from burn out and it sounds like you might need help in the future dealing with your brother when his perspective helps prevent you from jumping down a rabbit hole.

Anyway my advice is not to simply run the other direction here, it's to have an open discussion with the person you say you want to build a life with and figure out if that's really what you both want or not. If it ends up not being the right thing for both of you maybe you can part in a healthy way rather than feeling guilty about family the obstacle. I hear two things in your OP and responses here, I hear that you really want to build a life with this man and like being considered a part of the family on one hand and then that you are looking for an out on the other. Discussing it all with your partner is going to be the best way to make it all clear and make the right decision.