Legal Question in Wills and Trusts in California

If the house was given as a valid gift prior to his passing it does not pass as part of his estate and your mother takes it clear of any claim by her sister under the will.

Does the lawsuit allege undue influence or financial elder abuse? No matter if the gift is valid or not, if your Mother has been served with a lawsuit she will need to defend the lawsuit or your aunt will proceed to get a default judgment against your mother. You should have your mother meet with an attorney who can review the specifics of the lawsuit and she should do so immediately before any default occurs.

Based on what is the sister's claim?? Just that she doesn't like it? If there is a lawsuit, it has to be dealt with quickly so no counter-claims or defenses are lost, as mentioned. But if it is sour grapes because Dad gave it to one daughter over another, the case won't go far. If it alleges other things like fraud or duress or breach of contract, there may be more to it. As the house was not in "the estate" when Grandfather died, it was a gift to her and absent undue influence or the like, it is Mom's. Period.

I think you get the sense from the other responsesthat your mother needs to visit a lawyer as soon as possible to defend her claim to the house.