Which son for Christmas?

Stop worrying and go , your son with the grandchildren will fully understand and TBH ( in a nice way ) your GC will be far too busy to miss you.
Enjoy time away with your other son as I am sure he will appreciate time with his mum. We never know how long we are on this planet so go and enjoy why you can . Explain to your son that he needs to carry your bags , but I’m sure he will anyway , this will make the journey easier for.

Make some memories and next year try and get all the family together ( including your single son) , it’s only one day.

I’m spending Christmas Day with my step daughter , daughter , son and all 4 GC (Oh and husbands ex - wife of 30 years ) so if we can do this for one day for the GC they will have fond memories. Good luck x

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.