Okay ladies

I have a 5 year old step daughter and we have full custody of her. For the past month she has been so bad. She’s been pinching and punching kids at school. She also stole the other day. She comes home and gives me such a hard time. I put her in timeout and she screams like I’m beating her. I don’t yell or hit her because she isn’t my child and I don’t discipline my little one like that. We had our little one about four months ago and she’s a little rough with her as well. We make sure we give our 5 year old a lot of attention so she don’t feel unloved. Our problem is when she comes back from her moms she’s just terrible. She goes with her mom every other weekend. She was such a good girl and always listened to me and now she doesn’t. How should I discipline her ? Anything will help.