Maybe you should be staying with her instead. To me it feels like she is not comfortable at her mothers place. That is why she behaves different. If she is not giving you any problems then take her in. Maybe in the other side she does not get the attention you give her. Sometimes you have to note that children tend to want attention. Like they should feel they are the only children. This is why there is a lot of sibling rivalry when they are young. But when they grow they leave that aside. So she feels she is not getting that attention at her mothers place. Since there are other kids too. So her mother has to balance the attention. But in your case it is different. When she is with you she gets undivided attention. And she likes it that way. So just stay with her till she outgrows that behavior. When she does she can return back to her mothers place.