!!TW!! Does it sound like possible abuse?

My son is 2 and my brother is 22 with Asperger’s. He offered to babysit on the days that I work. It went fine for 2 months but then I could tell he would get mad about having to do it (like slamming doors or would yell at them for crying) so I started asking my sister instead. Well my sister let me know that she asked him to watch them for 15 minutes while I drove to get them. Today, I came home and my son has a few small circular cuts in a line on his lower back and there was a little bit of blood. He didn’t tell me about it beforehand but I saw it and asked him. At first he said that he went upstairs to grab his headphones and my son had it when he came back, so maybe he fell in between then and didn’t see it. I let it go since it didn’t seem like a major deal. But then I noticed that he mentioned another spot where he claimed my son “fell” and that spot didn’t even really make sense to me. Then later on, my other sister came over to me and said he was lying and showed me a text thread where he actually asked her to grab his headphones for him