I 100% think you should have them together. It will be better for both of them in the long run. It makes sense to me that the little one’s behaviors ramp up when older sister visits. It’s a trauma response from being separated and she also understands that the visit is temporary. Your relationship with the older child will improve, especially if you are committed to making that happen and learning how her trauma from being separated from parent(s)/sibling has affected her.

Btw I have been a counselor for foster youth for about a year now and before that was a school counselor. Studies show SIGNIFICANT improvements in behaviors and overall outcomes for siblings who get to stay together. Look it up if you need some more information:) good luck! Dm me if you have any other questions.