Need Advice -2.5 year old, doesn't talk much, kicked his 7month old cousin

This is my first time posting anything and I'm super desperate for some help/advice/reassuring words...this is going to be a bit long.TLDR: My son kicked my 7 month old niece and I don't know what to do.My son is 2.5 years old, and this is about how he kicked my 7 month old niece.A little back story... First off, my son does not talk yet, several words here and there, like eggs and mom/dad, but not sentences. My SIL (the mother of the 7 month old) is extremely verbally aggressive towards anything she sees as "bad". This extends to not only her child, but mine as well, also ANYONE around that is doing something she doesn't agree with. Also, I live at my in laws atm because we are moving out of state in the next 5 months and we needed to save money, we pay rent so it isn't like we are freeloading. Since the neice was born my MIL shows her much more affection and love in various ways, we have been living here for 11 months now. This is the second time this sort of situation has happened. The first time, (I was not in the room) my son attempted to kick my niece and my SIL grabbed his leg and yelled at him. My MIL later tells me that this happened. I told her that I don't want my SIL to touch my son in any aggressive way, it is not her place and I do not hit my son so no one should.Start of tonight's story: My husband and I are outside, my MIL was throwing a little part for xmas so we were hanging out. My son was in the back yard with our 9 year old nephew and I didn't notice that he made it in the house. I have already told my husband about the first time and told him not that one of us has to be around at any point that my son is in the same room as my niece. Since neither of us knew he made it in the house, neither of us were there. A couple minutes pass and my sister in law can be heard from outside screaming "I'm taking you to your mom" she comes out hysterical telling me my son kicked her daughter. My husband says "did you hit him?" She says "no, I didn't touch him". So she looks at me and starts going off about it and I said, he is 2, he is still learning. She flips and say something about he shouldn't hit and I told her I understand that but at this point it's already passed I don't feel right reacting negatively to him when he probably won't even understand why. So anyway both of us are pretty upset and neither of us are in the correct mind for this conversation, but it persists. I said I understand he shouldn't hit, she goes on about how I "snapped" at her mom about her touching my son so she brought him out to me. I said good, that's how it should be. Anyway the whole point of her bringing him to me was lost because I'm upset that she's screaming at me and she's upset (rightfully so) that her daughter was just kicked.I have no details about why he kicked her, how hard he kicked her or anything else, just that she yelled at him, he is crying, and she's pissed.I have NO IDEA what to do next. About my son kicking, how to stop it, how to discipline or how to smooth things over...which part of me doesn't care to do because excuse my language but she's a royal bitch to everyone to begin with.Please help. Ask any questions if needed. Sorry for long post. via /r/Parenting http://bit.ly/2SiqooJ