A Maybe Solution:

I think, the answer to this problem, might be very simple, if a bit deceptive.

Why does sister have to know brother and his family moved into dads? Can’t we just not tell her? The move in is apparently temporary – though I believe they will be there much longer than dad thinks, but, BUT, sister rarely goes home to dads, not for lack of want, but because she hasn’t got the transportation.

Can we just not tell her whats going on?

This, it seems, is out game plan to avoid hurt feelings. And? I’m pretty for it!