S.B.
This is a hard post to answer because there isn't much information. What is the age of your daughter? How long has your cousin had her? Why did you give her temporary custody? What is stated in your agreement?
That aside, what is in the best interest of your child? Put aside all bitterness and everything, but think about your daughter. I don't understand why you want to switch your daughter to your mother. You are talking about a lot of change in your daughter's life. It would be much easier on HER is she either stand with your cousin or went home with you.
Again, think about your daughter. Put her needs first not yours. She isn't some kind of toy to pull back and forth and say "mine".