dazed, why is your sister and her family living there without paying rent? I understand her husband is injured, but can't they pay ANYTHING? Can she get a job? If they had their own house and a mortgage, they would have to pay it, if they were renting from someone else they would have to keep paying, can't they even pay some rent and catch up on arrears later?
I'm sorry you are going through this, I don't have much advice for you, but this point jumped out at me...