4 year old violent behavior

Before I say anything imma note that I’m practicing gentle parenting with my nephews but my sister isn’t a gentle parent. I’m making this to ask if there is anyone that I can call to get my nephew seen by a behavioral therapist I told my sister that he needs to see one because the way he acts isn’t normal and is not acceptable. Here’s some examples. I was babysitting the last 2 weeks and I have a 6 month old baby, i have 2 nephews ones 4 and he has an older brother that’s 8 his brother has ADHD and is on the patch on school days so I was only watching the 4 year old most of the day and his brother would come home from school maybe an hour before their mom would get off work.

1. When I would put my baby in his bouncy chair he would bounce him soft at first then the second I look away he would bounce him so hard that his head would come off the chair and I showed him what shaken baby looks like (he wasn’t phased) and told him “you’re going to hurt your cousin if you bounce him that high so I’m going to have to not allow you to bounce him anymore if you aren’t gentle”

2. One time I laid my baby on the couch and he comes over the arm of the couch and steps literally on his chest with all of his weight and I said “your cousin doesn’t like when you step on him he’s crying because he’s hurt” he said “I like hurting my cousin” with a smile on his face. At that point I had no idea what to even do so I tried my best to not leave my baby laying around because I knew I couldn’t trust him around him after he said that.

3. He ask me for a chocolate Popsicle when I had just made lunch and when I said he had to eat his lunch before he could have a Popsicle he literally started SCREAMING at the top of his lungs “BUT I’M NOT HUNGRY” and not even crying he was just screaming he does this all the time over any minor inconvenience like if his tablet dies it’s nobody’s fault but his but he will start screaming and get mad at everyone and start throwing things and hitting his brother who didn’t even do anything then proceeded to hurt my baby if I’m not right there to stop him. Anyway he literally screams as loud as he can and says “I’m not going to stop screaming until I get ___”

4. He says “if you don’t give me ___ I’m going to ___” things he’s threatens to do are usually “Break all your stuff” and “hurt my cousin” he’s also said he’s going to “kill” me, his brother, his mom, my baby, basically anyone that doesn’t give him his way.

There’s a lot more things that he does that concerns me but I can’t think of everything right now. Thanks in advance please no hate.