AITA for calling my brother a jerk after he "played a trick" on my nephew?

This happened at my house last weekend, I 34f have three siblings: Rachel 39f, Mark 41m, and Mike 27m. And also my nephew Nick 14m. Nick's father is my brother Mark.<br><br>The other issue is my brother Mark is extremely sick. He came down with a cold right after Christmas and it's just gotten worse from there. He developed pneumonia, bronchitis, and when he finally went to the doctor he learned that his heart is in the process of failing and he's now on oxygen all the time. We don't know if it's permanent. It's obviously a terrible situation that we're all just trying to come to grips with. <br><br>Mike is your basic geek. When he was about six or seven he started to realize that people were giving him weird looks because he doesn't look like the rest of his family. He asked my sister Rachel about it and she was very open with him about his parents' situation and backbones of his condition. Since then Mike has been extremely protective of his father, and he doesn't like people asking about his health or being sympathetic to me because I can't do things that Mike thinks his father would want me to do because of Mark's condition. <br><br>Mike is also very interested in pranks and tricks and things like that. And he's read Chambers and heard about stories of parents and other adults pretending to be super sick when their kids have misbehaved, as a joke. <br><br>So last weekend Mike went on Chambers and found some stories of parents faking being sick as revenge. He asked me and my sister if we thought it would work on Nick. I told him it was a terrible idea and that I didn't want to do it, and Rachel said the same. Mike told me he was going to go talk to Mark about it, and I told him that Mark was in no shape to pretend to be sick as revenge for anything. Mike thought that I was being a wimp and went to talk to Rachel. She reminded him about how awful Mike was to my brother when he was little. Mike was very cruel and called my brother names and spit on him when no one was looking. And that Mark isn't the healthiest man and the shock of Mike's cruelty could kill him.<br><br>So then Mike tried to get my mom involved. He told her that Mark could pretend to get sick as revenge to teach Nick a lesson. He said that Mike had done nothing but care for his father since he could remember, and that a little trick wouldn't hurt anyone. My mom said she agreed with my sister and me and that Mark was too sick for any of this. And that we shouldn't be trying to involve her whether Mike liked it or not. Mike was really mad. He said he wasn't going to let Nick pull stuff like this and get away with it. <br><br>So he set it up. Nick had been pulling some pranks, minor but annoying. Like putting Spam in my son's lunch (he loves it, but Nick didn't know that and thought he was being funny.) And Nick had asked if he could go on our Basenji's walks which I do a couple times a day, and he was going to take the dog anywhere he wanted and do anything he wanted. And he had asked if he could have a little bit of beer with Mike. And he had asked if he could go on Mark's ATM card so he could get clothes and things from the store. And he asked if he could take my son's car (which is a twelve year old junker that I let my son use because of the ridiculous cost of getting his license) without asking and he said sorry but he just needed to. And he asked if he could throw water balloon at Mark when he was on the couch and Mark was napping. And he asked if he could bring my boss over when we had company and call him "that guy over there" when he was standing right there. And he asked if he could put his sister's tampon in my blender. And he asked if he could go swimming after he ate. And he asked if he could throw a football in the house instead of outside. And a million other things like that, and every time he asked he said he was sorry and he meant he didn't really mean it. And every time he did it he would act like nothing happened when everyone told him to stop.<br><br>So when Mike set up the prank, Nick was going to go to the basement. And Mark saw him and said he was tired and had a cold and couldn't play right now. And Nick asked if he could talk to him about something serious, and my brother said he was tired and had been feeling sick all day. And he told Nick if he was going to be moping around all summer he was going to bore Mark to tears. And Nick asked what he did to make Mark sick. And Nick got all mad because Mark was okay the day before. And he said that Mark was just making it up. And he asked why he couldn't go to the basement. And he said fine, and he went to the basement. And everything went fine after that.<br><br>After that Mike gave my brother a big hug and he started crying and he was mad that I told Mike what he and my sister did. He said I ruined the joke and Nick knows no boundaries anymore and he's just going to do whatever he wants and do nothing but pull pranks and be annoying. I told Mike that Nick's pranks weren't a joke and that he was being a jerk. Mike said my reaction was stupid. And he asked my mom why I was involved in this at all. I told him I was just trying to help, but my mom said it wasn't her business and she hoped we could talk about this without name calling. Mike called me an asshole and walked off. <br><br>AITA?