Odd behavior

So my step daughter that is 9 is now showing some weird behaviors we’ve never seen before. So I also have a 2 year old and a 9 month old but back at her bio moms she’s the only child. But the other day I caught her stepping on my 2 year old the other morning to wake her up.. mind you the 9 year old is like 80 pound my 2 year old is just barley over 30 pounds. She ignores my two year old when she’s using her words such as “sissy I have some slime please” and the 9 year old will straight up ignore her playing with the slime until she starts crying because then she knows I’ll look on the camera to find out what’s going on.. then today I was giving my 2 year old a bath and also on the phone with my grandma and some how she found herself in the bathroom with us. But she was splashing my 2 year old in the face, my 2 year old is asking her to stop and is clearly struggling to breathe and the & year old is just laughing about it and ignoring the fact that she’s asking her stop.. to me it seems like she’s bullying her idk any tips and experiences like this…