There might be other reason

Assuming most people at most time are kind and reasonable, if someone is acting unreasonably, first ask why they behave out of the norm.  I don't know why the father and the sister is not considering any alternatives, if I was there I'd suspect some kind of back story, esp. the sister won't talk to the mother for 10+ years and father is overly generous.   Like Old Cat said, you can't ask other's problem for them.