At least you're not trying to decide which one to spend Christmas with between two sons who don't get on!
Our daughter and older son had a major falling out recently and really don't like each other. Completely different personalities. Breaks my heart but it's time to stop trying to paper over the cracks. (The younger son gets on fine with both his sister and brother).
This year we're not due to spend the holiday together anyway but I can see it's going to be a problem in future years: she has her own house, a partner and our only grandchild, both sons are single with nowhere else to go.
I vote for going to visit your sister, while you can. You will have lots of exciting stories to tell your grandchildren when you return.
I've got another 'keen'... Ouch!