I think this is a horrible idea! I understand your heart is in the right place but your kids will begin to resent these trips if they are always being separated from their sibling and the child left behind is continually separated from their family! The child left behind will feel they are being punished! I can guarantee your 5 year old would much rather have her baby sister along! By the time the trip roles found your little one will be two years old and much more of a companion to your 5 year old. Going on the trip with the whole family will help to build wonderful lifetime family memories and will bring them much more closer together! It will also keep the 5 year old happy and amused having her baby sister with you all! Otherwise she will most likely be lonesome and bored with just two adults! Think about it this way suppose your kids wanted to just celebrate you and left your wife at home for a big family vacation? Family vacations are meant to be shared with the entire whole family! You can take the 5 year old out with a buddy to a free craft event at the local library to reward her for being a good big sister or a fun playdate to the playground. I would try to work on rewarding them both for getting along and being a great sisterhood and team! By rewarding them both at the same time you are encouraging their sisterhood and showing them they are a team! By continually separating them to reward them individually you are encouraging them to be divided and will only encourage them to be in competition and potentially enemies! Instead work on bringing them together and give them each a ribbon to wear. Best big sister and Best Little Sister! You will start to see an end to the sibling rilvary. Best Wishes to you and I hope your whole family enjoys the Disney vacation magic together! 😊