On the one hand I would hope your sister could understand that, while it's disappointing, it's not in the kids' best interest to throw this change at them. On the other hand, not all disruption is bad. There will always be things that occur off-cycle, so to speak. It will be up to you and your ex to decide what's worth accommodating. Is your 6 1/2 year old on the mature side, and could you ask them whether they'd like to go to their auntie's party to celebrate her birthday? Or will they just tell you what you want to hear? One of my nieces was around 9 when my husband and I got married, and her mom/my SIL decided the family couldn't come because of a schedule conflict. We learned years later that the niece's feelings were hurt because she thought we didn't invite her to our wedding. Sharing this to let you know you might want to manage that with at least the 6 1/2 year old, in case they hear about the party later and have similar feelings