Help, our daughter's biomom is alienating us and it's working

My boyfriend has a 4 year old daughter. Last year in July, bf and I ended up having to move for work, and our child stayed with her mom. We always do our best to make sure she is happy and healthy and she knows she can call us at anytime. We visit as often as possible.On one of our visits, our child mentioned that sometimes she wants to call but "mommy doesn't let me. She says daddy is busy." I know my bf would never be too busy for her, but when confronted the mom said she was lying.Before we left, she was spending about a week at each of our houses (ours and her mom's). For the longest time, we didn't know where she lived, just that it was a trailer with an uncle of the mom and her mom's three kids and boyfriend. We still do not have an address or phone number for them but know they moved recently into a house. Before when they were staying in the trailer, daughter would come home to us and tell us things (unprompted) like "Mommy and Daddy Z can't afford to pay the rent. Mommy and Daddy Z have a lot of sex. Mommy and Daddy Z fight a lot.  Sometimes Daddy Z hits Mommy and doesn't say sorry!" Those are concepts that 4 year old children generally do not know.(Some quick back story. Her mom basically abandoned her on her 3rd birthday to fly to Cali with her other two kids from a different dad and cheat on my boyfriend, her husband at the time. Not only that but the guy she was seeing happened to be my bf's best friend. She came back into daughter's life a few months after, acting like nothing happened.)Her mom is always feeding her things about me like "she's not your real mommy" etc. This got to the point where daughter started believing this, she took to hitting me and wouldn't stop, all because I took away her gamepad for being mean to the animals and not listening to me. I was very clear with her, "you are not being kind to the animals. You can have it back when you decide to be kind." At this point, I go to the kitchen and place her gamepad on top of the fridge. She starts crying, and then starts punching me in the stomach. I tell her to stop. I ask if she needs something (in case she was being grumpy because she was hungry or another need wasn't being met). I try (gently) pushing her away. After a while, she finally settles down and I give her a snack. I ask her if she does this to anyone else, she says no. She is a great kid, usually very well behaved, and she calls me Mommy [Name]. I treat her well, she is adorable and like I said well behaved, so I rarely need to punish her, and I never use force or hit her.I need help. The mom is clearly using our daughter against us, as a weapon, as a tool, and it's not right. Not to us, and not to our daughter. She deserves love, not manipulation. via /r/Parenting http://ift.tt/2pxtzOx