Well, the person who's really going to be whacked the hardest in this is Gwen's daughter. Fight or no fight, changed mind or not, she didn't have a hand in any of this.
Maybe you, your lawyer, the lawyer for the estate, and a guardian representing the daughter, with the daughter present (but not all the time) should sit down together and talk about the girl's future.
Maybe the girl has other ideas in mind. Just as her mother knew she was dying, the girl probably had some sort of inkling (but I caution you that this is a minor so we can't ascribe mature motives to her).
But there may be a cousin, a friend's family, etc. Or you and your husband may find you'd like to be the girl's guardian after all. Or maybe the girl says she doesn't want you to be her guardian and would prefer foster care (again, this is not an adult, so she will not necessarily be making the best decisions for herself. This is why children like her get what's called a guardian ad litem
So, you'll see. I think you'll feel better about things if something like this happens, even if you don't take the girl in. Just washing your hands of it would be easy, sure, but again, none of this mess is the kid's fault.
You can still say no. I'm just suggesting that you face the girl if you do.