Hi I can't answer your questions as it's not clear from your post which cancer he has been diagnosed with prostrate ot pancreatic. However, as to your niece and as an ex safeguarding social worker your best bet is to contact children's services and ask for advice. Your father could and would probably be better to consult a solciitor specialising in Family Law (financial implications as it's classed as private law).  This is a bit premature as he hasn't had the full results yet. However, I urge that your niece's legal position is sorted sooner rather than later. Sadly, if it isn't social services will become involved and create a lot of unstability for your niece's future. So sorry I can't be more helpful