Their is always supervision, i they play just like any other kids would, they play outside riding bikes, playing with toy guns, or basketballs, and we are ALWAYS around. She sometimes goes upstairs into my nephews room and plays with his toys, but their are tons of other kids up their and i am always checking on her because well she is 4 and still tries to get into things she shouldnt, like my sisters makeup and stuff lol. Most of the time the kids that are always their are my nephew who is 10, the neighbor had 2 boys i think the older one is 8 and the younger one is the same age as my daughter. My nephew is very popular and his friends come over alot, but they are usually running around outside, riding their bikes. I did read something online that said kids should be going to the bathroom 5-7 times a day and that can be an issue if they arent and well my daughter doesnt go that often when we arent home.
Is there supervision when your daughter is there or are the children left to their own devises, how many children at the house and how many parents/adults?
Some children are uncomfortable if they are not in their own beds, or using their own bathrooms. I had terrible trouble with one daughter who would not go to the bathroom at school, even though the school was very clean and all the fixtures child sized. Eventually they outgrow this sort of thing, but it can be a trial.
I would suspect she is stressed or distressed about something, over at your sister's, if she is able to maintain potty training everywhere else. I'd stop hanging at my sister's if this is the result. Even if it is not sexual, the boys might make her feel anxious in some way.