First of all when u get married tell the girl and her parents in presence of mediators very clearly that you would not live separately and in case your wud be wife stated so u wud break up the relation.
Regarding your bhabhi I think u or some one close to her need to ask her about the reason of asking separation. If she honestly tells and you can address her grievance than everything will be fine but in case her demand is just foor desire then your family needs to treat her strictly by conveying her parents that whatever she is asking is not going to happen.
By the way the cruel truth is that every family gets separated one or the other day and what matters is not separation but relationship. I have seen families where boys living separate from their parents do take proper care by visiting them twice a day. So, If your family agrees you can either make two portions or ur brother may shift to other or rented house. Since I don’t know your financial position and other details I can’t comment precisely.
Your family can also tell her in strict sense that if she gets separated they will not get any money from your father and they need to manage on their own.