sorry, but i think they have every right to ask you to leave (lawfully), and the fact that they have waited 2 years probably means they are not "money grabbing"
in the nicest possible way, are you expecting them to "look after" your mentally and physically disabled sister if you died? At 65, she could live another 30 -35 years, are you expecting them to wait this long before selling? was there anything in your sisters will about what would happen to the house (except it would be left to him?)
You would be better off not looking at this as a problem of your nephews making, rather your late sisters making im afraid