I need help ladies, ADHD kid and unpleasant mother

I Need Mothers Opinions Please If You Have The Time Read The Whole Post. My cousin came to me and told me his wife was pregnant, which i was shocked because they're 4 year relationship has been very rocky but whatever. Nobdoy in the family was really happy and she doesn't have a relationship with her family (her mother was a drug user and gave her up to child care services) the rest of her family is in her country who she never really met. She has a daughter whos 10yrs old from another marriage and i obviously knew her but didn't spend much time with her cause she was always with her babysitter. So the babys born but covid happens so shes suck at home and I'm working over time due to shortness of staff. 3 months go by before we finally see eachother again, she comes over but only has the baby. She tells me it was really hard having the baby and the 10 year old who has ADHD by herself, she tells me my cousin and her aren't doing well so she sent her duaghter away. Ok fine didn't think much about it. Then as we become closer again she tells me that her daughter went though a situation with her babysitter husband in which the little girl states he touched her but she didn't know what to do it became to stressful to deal with the situation hence sending her away. So at this point I'm like WHAT? OK fast forward to now, its been about a month since her daughter came back and i see some behavior that doesn't seem like ADHD

ex- i have cameras in my bedrooms which my kids turn off when changing ect but know to turn them back on i guess she was under the impression they were off and i see her look out the room door then take a blanket and cover the baby and laided on top of him

Ex- camera in the living room she took the baby out the high chair sat him on the couch and walked away the babys 6 months so he fell forward onto the floor shes then seen picking the baby up off the floor and covering his mouth to get him to stop crying

Ex- camera in the kitchen my 3 year old is bothering her and my older daughter i see her turn around and what seems to ask him to leave her alone at that point i walked in and took him with me so they could finish eat at some point while I'm cleaning he goes back into the kitchen and she looks down sees him there and swings her feet back hitting his face and leaving redness, she swears up and down it was an accident....

Ex- i decided to take them to the park to let off some steam so while there my 2 year old is trying to help her moving this toy at the park she gets annoyed by it so she trips him, he started crying she ran away, as if he just fell and she had nothing to do with it....

Now i understand ADHD I've worked with special need kids and it also runs in my family but this behavior and others don't seem of a kid with ADHD and her mother is aware of all of these situations and all she does is hit her and then tells me its her condition and that shes on her medication and seems like she needs a higher dose. I've expressed to her many time that i don't believe hitting her is going to fix the situation and that i understand shes overwhelmed with the baby, her daughter, having to stay at my house but being depressed and not dealing with the issues isn't going to make it go away and that i believe she should get her evaluated again because i don't think she has ADHD alone something else has to be wrong.

I included some back story so you all have an idea of the unstableness they have, i don't know if its the little girl lashing out or what but i need advice on how to approach this situation moving forward with them staying here and a good way to bring it up to her mother because I'm truly afraid for the baby and my kids