So it may be different in your state, but I owned my house before my ex and I got together. I paid every payment from my own account, and we never co-mingled money in it.

My lawyer (correctly) advised me that he was not entitled to anything I owned before we got together, but if we had mixed money he would have been entitled to half the increased equity.

However, since the house was purchased before they got together and your stepmom only owns half, best case scenario, at least in my state, would be the increased value of 1/2 of the house, and only if they co-mingled money. Since Dad paid rent to Stepgrandma, I don't think that is the case. It establishes a tenant/landlord relationship.

If there is a second mortgage on the house, it is most likely in both stepmom and stepgrandmas name. Dad isn't entitled to any of Step Gma's wealth.

Definitely meet with a lawyer, find out Dads rights. If you don't like that lawyer, interview another. I think SMom probably had her ducks in a row before the split. Good luck.