2 year old smacked in the mouth

Okay so for some reason my 2 year old has been biting very badly. It just started a couple months ago. And she hasn’t been around anyone that even bites. I tell her no that it hurts, she doesn’t do it to adults only kids and she only does it if you aren’t in the same room or aren’t looking directly at her. Even tho I tell her no no she continues to do it. My baby father has a gf and they have a newborn together and my baby father has sent me multiple pictures of bite marks on the newborn and I do feel terrible but idk what to do. They said they can’t turn their back for 2 minutes without her biting the baby. 3 hours after I drop my 2 year old off at their house ( she stays every weekend) I get a call from my baby daddy saying his gf called him while he’s at work crying saying my daughter had bit the blood out of their sons cheek. And she apparently had enough saying it was the second time that day and smacked my daughters mouth. I’m infuriated. I called this girl and asked her why tf she’s putting her hands on my child and we got into an argument and her saying I baby my daughter and won’t discipline her and that’s why she acts the way she does blah blah. Anyways we ended up talking civilly she said my daughter had fell asleep on the couch and the baby was in the swing so she went to the kitchen to do dishes and hears him start screaming and he had a bit mark and blood on his cheek and my daughter was standing beside his swing. And she said she didn’t smack her that she “tapped her mouth” and put her in time out but for some reason I just don’t believe that. I ended up picking my daughter up and just keeping her this weekend until we get this sorted out. I don’t even like the fact they put her in time out I don’t feel its right but for that bitch to put her hands on my child. ugh I’m pissed idk what to do

Update: I didn’t think this many people would be commenting.. lol but i did have a sit down with baby dad and gf and she agreed she doesn’t agree with hitting children she claims she done it out of fear and frustration and we all agreed physical punishment won’t be our route. She went back over this weekend and she didn’t bite this weekend. So maybe the smack in the mouth did scare her out of biting but I’m not so sure it’s over yet. We did schedule and appointment with the pediatrician and he literally told us to bite her back not super hard but every time she bites do it back so she knows how it feels. Idk how I feel about this yet, I’m going to do some research but they referred us to a psychologist and we will be going there in 2 weeks. So I’m hoping this will help. Bd gf bought baby gates to keep them separated when she has things to do since she does keep her while my daughters dad works, so we are hoping this will eliminate some of the problem as well. And as for time outs I don’t agree with them and won’t do them at home because i don’t think it’s right to make her stay in one spot while she screams her head off wanting down, it seems irrational to me but her father wants to continue time outs at his house. I think my form of punishment when she bites is just redirection, like I seen one comment say to tell her “teeth are for eating not biting” but thanks to everyone for advice !