If I am divorced and my ex-husband dies, and I survive, does he have the right to give full finanical guardianship of our minor son to his sister?
This depends on the standing custody agreement. With financial responsibility, presuming you are not under a restraining order, or haven’t relinquished your parental rights, and that the child is in his full custody, you would likely be next in line to be explored. If his last will indicates that he wishes custody to go to his sister, this will be highly considered in a court case.
My best advice is to make a stipulation in your custody agreement at this time, if possible, that if something were to occur (even outside of death) where the father is unable to continue to care for the child, financially or otherwise, you would have the responsibilities remanded to you, or would have the right to pursue that in court. It is my understanding that you have the right to attempt to pursue custody again at any time, unless otherwise ordered in court.
Best wishes, I hope this was helpful.