Well done Gloria. You sound so clear headed and sensible. I'm sure you'll get through this.
Part of the beauty of life is that we are all different to a certain extent. Some people prefer their own company during the early stages of difficult times and others prefer the company of others like strangers.
Gloria/OP do what works best for you. Your sister-in-law sounds willing to understand that it would be good if you attended as planned.
Can you speak to your husband to come to an agreement that even though the relationship is coming to an end that the two of you an act civil enough to get through Christmas day for the kids?
Is your husband still living at home with you?
You sound like a really level-headed mum. Keep putting the kids interest first. I hope your husband can do the same. Since you both love the kids, if you all put them first the two of you would masfiveky increase the chances of the two of you getting on well despite being separated.
I am sure you'll get through this. If you want revenge, show your husband that you are holding your head high and moving toward.
Positive thoughts and best wishes coming your way. It will be hard and there will be a lot of sorrow but joy will come. You must hold on to that truth. You will be happy again.
Use this as an opportunity to do nice things for yourself. If the kids are spending time with him in the future, go do your hair, nails, etc. Take up a sport. Do the things you always wanted to do but never had the time. Use this as an opportunity to live life.