S.W.
With all due respect, your mom needs to keep her opinions to herself. If the girls seem happy to you, then what should it matter that one day they said they lived at their Dad's? It's not a competition. It sounds like you and your ex are handling this well, with your children's best interests at heart. If you don't think the split week is a problem for them, then you know best. Just keep listening to the girls. It sounds like you are both willing to be flexible if it seems like their needs change, and they probably will. I'm not divorced, but my parents were, so this issue kind of hits home. I did every other weekend with my out-of-home parent. Which was fine until I got into my pre-teens/teens and I wished it could be more flexible... because I missed parties, etc... because I had to be "at mom's" and couldn't bring myself to hurt her feelings and say I wanted to switch things up. I would recommend that you just keep listening, keep being flexible as they grow. Try to keep your feelings out of it as best you can, because although my parents did the best they could with the situation, there was always a lot of guilt on the part of us kids (not wanting to make one parent feel "second best"). Good luck... sounds like you're doing great.