Why is your husband mad at you if it is his sister who is cancelling each time you set up a time? Anyway, it is frustrating to deal with all the family stuff if it isn't set up from the beginning. My husband and I have always spent Christmas eve with his family and Christmas day with my family. It has worked out very well for us. My son and his wife are on the move more. They go to Christmas eve at my husbands parents then go on to my daughter in laws grandmothers house which is about a hour and half away from my inlaws. Then they spend Christmas eve night here at my house even though we live in the same town, so we will be able to watch our little granddaughter surprise with Santa (which I buy the majority of to help them out) and then they go to my parents to open presents and visit a little then head over to her mother's for Christmas dinner.
One thing I would want to mention is that it isn't worth arguing over. Your husbands sister is sounding a bit flighty and you can't always plan ahead with her, but maybe you could talk with your parents and maybe she and her husband and kids can come to your parents for Christmas dinner, leaving them to do the traveling. Or set up a time for each year and if the plans get cancelled, say sorry but don't worry about it, the road goes both ways and they can come see you just as easy.
My daughter in laws step father had passed away this August so this is a hard Christmas as it is for her mother and of course for her also since he was more of a father then her own. Her mother went all out for Christmas this year trying to make up for missing their husband/father/grandfather. Then Friday, the 19th, her mother's home burned down to the ground while she was gone. They lost everything, the house that my daughter in law grew up in that her step father built with his own hands, all his pictures, all the baby pictures, everything including all the presents that were bought. As sad as it is and as hard as it is, we all know had she been home she would have ran back in to get things of her late husbands or the presents and we could have lost her also. My son, daughter in law and granddaughter was suppose to be there Saturday night for a birthday party (yes, the house burned down the day before her mother's 50th birthday) and they would have been sleeping in front of the fireplace which was the cause of the fire.
So my advice here is simple, don't argue over things that you cannot control, instead appriciate that you have family close enough by to enjoy the holidays with. It doesn't matter if you spend 1 and half days with your parents and 1 and half days with his sister or all 3 with your family, seeing his sister when you can, at least you are all together, you, your husband and your children. Be blessed with that.