Ha ha!
For my Dad it is like siding between his Mother and his wife. So on one side he has to please his sister and on the other his wife.
Dad was very emotional to be truthful. But I am worried for him as he really seemed so down in the morning. He told us Phophoo phoned him, then he went to work.
You're the oldest and wisest. Tell your dad that a wife and a sister are two different relationships and that they both CAN'T be compared and they both can't be replaced. Each person has their own unique place in the family. Tell this to your dad and tell him to tell it to Phoopo.