Help, Tips, Adivice?!

I have three children of my own ages ,4,2, and 1. I also babysit a child almost age 2. The child I babysit will constantly take my 1 year olds cup even though he has his own. Both cups have the same drink in it and stay filled up. This child will purposely go and take my 1 year olds cup and empty it and then drink his cup. I have tried so many things to try and stop the behavior, but nothing seems to be working. It gets a bit frustrating at times because I’m at a loss. My one year old doesn’t take his and gets upset when he empties hers. Any ideas of ways to stop this?