I agree with the above suggestion. Also, any change/transition is tough for our kids. I'm not sure if your daughter understands that her sister will be leaving and coming back again. Maybe use a calendar and write on there the schedule so that she can see when her sister is leaving and then show her when she comes back. Since our kids are typically very rigid you can use that to your advantage in the instance and let her know (for example) "sister leaves on Sunday and comes back on Friday" and that is the general "rule".

Perhaps you could do a social story? I've seen people on here that say use stick figures and a photo face. Create a story to show how her sister only comes during the weekend and how your daughter can act. Like give a big hug and kiss to her sister to show she will be missed. And how her big sister will come back again. Then read it to her so she knows how to act.

I have a 13 year old stepson we used to get every other weekend. My oldest (11yo) with autism used to have meltdowns every time we would take him home. He would cry, sob and be unconsolable. We tried distracting him when it would be time for Jer to leave or we tried having Johnny go out with grandparents just before Jer would go. Sometimes that worked sometimes it didn't. Jer now lives in Virginia and we only see him a couple weeks in the summer. Johnny 'forgets' he exists until a visit and then the meltdowns when he goes home are horribly long. We still try distraction and picture books with lots of pictures of the two together.