Family wanting baby

El

My daughter is 21 months and my husbands family keep asking to have her. One of her aunties is 20 and she keeps on asking. We have allowed it in the past but I dont feel comfortable anymore because I have seen a different side to her. She works with children so I dont feel like my daughter would be in danger but its more how she treats my daughter. She will often snap at her in front of us or do silly things to wind her up like get her totally soaked with water and things like that. I feel like if she does those things when Im there then how is she when Im not. and my daughter is too young to be able to tell me if she has shouted at her or smacked her etc. So I just dont feel comfortable with the whole thing anymore. My sis in law didnt used to be like this hence why I was ok with it before. But I dont know what reason to give to say no she cant look after her. Any suggestions??