Step mom seeking HELP

My fiances daughter is turning 4 in a matter of months. Until she's in school, we drive a good distance to take her into our care for two weeks 4 times a year. We've never really used physical punishment until here recently (opting to use time out and 'groundings' from toys). She's been on a 5 day spree, something new everyday. The last time, she dedicated in her (nap time) pull up and was playing in it! In my bed! She tore the pantry and fridge apart while her and her father were supposed to be napping, and many, many other things. I've told my fiance she needs to be evaluated, but he has no authority over the matter other than telling her mother.

I'm at my whits end with this child! I've never been so upset, frustrated or guilty. How have other step parents dealt with children who behave like this?