Six year old doesn't understand

SD14 sees her BM one week a month and my BD13 is currently doing an every other week schedule between us and her BD.DH and I only have one child together, our six year old. A doesn't understand why her big sisters leave all the time, even though we've tried our best to explain the situation to her. She just wants both her sisters home, but obviously the custody schedule doesn't always work that way. She gets jealous when they leave and is often lonely when they both happen to be gone.Most kids would enjoy the 1-1 time with their parents, but not A. She throws a fit if we try to do anything special with her during that time and whines a lot.How do we handle this? I need advice! via /r/stepparents https://ift.tt/3kNnC8I