Am I being to rational or am I doing the right thing?

My name is Heather, I'm 25 with two daughters one 6 and the other one 2. My mother has 6 children all together me being the oldest, a 23 yr old sis, and 4 younger siblings ages from 6-13. I payed my mother to watch my children while I work, and on one occasion something happened between my 13 year old brother and my 6 year old daughter. I didn't find out till a few days later from my ex mother in law-when she stayed over night and the ex-mother-in-law brought it to my attention that my 6 yr old told her my 13 yr old brother kissed her and wouldn't stop. I was confused at first but only because she said it was my 8 yr old brother Tony and not C.J. at first. (C.J.) is my 13 yr old brother. I thought she was doing it for attention so I called my mother just to clarify- and I told her from my gut instinct I believe Tony wouldn't do that that maybe my 6yr old is lying-but just to look into it. My mother of course was furious and stated she never wanted my kids over there ever again....I stopped talking to her after that because she was being ignorant and instead of being an adult about it she said mean and hurtful things about my children. Well, I started to ignore it that maybe it was an attention trip for my 6yr old. But it ate at me. So last night I had a heart to heart talk with her about what had happened- I didn't feed any information to her I just asked her upfront questions- Me: Mina, what happened at grandma's? my 6yr old Mina: "Tony kissed me and wouldn't stop." Me: How? Mina: With his mouth on mine. Me: What else happened? Mina: He used his tounge? Me: why? Mina: He said he was just playing with me and wanted to try something. this was the whole conversation and I again asked her the same questions 10 min. later to verify she wasn't lying and not once did she change her story-this leads me to believe she isn't lying because a 6 yr would change her story if she lies I've seen her do many times before. Me: uploaded a picture of my two brothers and I asked her who did it she pointed to c.j. I said that's not Tony that's C.J. do you know who he is? Mina: yes my uncle. I called my mom to apologize about Tony and that Mina stated it was C.J. and this isn't the first time that C.J. has kissed someone like this. My own mother told me he did it to her, guess out of curiously and she spanked him for it. I was calm and told her before she flies off the handle there is something very important that I needed to talk to her about-I proceeded tell her it wasn't Tony and I knew it wasn't and I'm sorry,and that it was C.J. as soon as I went into detail she hung up on me. So I called her back calmly and said " I'm bringing this to your attention because this is important and DHS could get involved, I just want you to investigate and correct the problem before it gets out of hand. I'm willing to discuss this like adults but if you want to do this the hard way I'm more than happy to get DHS involved. My gut instinct is to call and report it-my little brother has anger issues and has put his other siblings in the hospital for violent attacks-and I don't play around with children being molested I was by my father for 13 yrs and my mother refused to do anything about it because she didn't believe me. My question is, am I being to rational or am I doing the right thing? I honestly believe that I am, by pressing charges-he needs to be taught it's wrong, and if his mother won't do it at then I guess the law will have to. I know how i would feel if it were the other way around, I would be mad but I wouldn't act in that such of manor. The thing is, that my family history has so many files of prison time for the men touching children in inappropriate manors. I'm here to break this cycle. Of course my mother and I are no longer speaking and I'm more than happy to keep it that way if it means the well being of my children. It has angered me to the point where I don't want to be intimate with my boyfriend, and am afraid that this might push him away. He believes my daughter too..but I don't know if it could be the rage that's causing me to be this way or what. That of course is a whole other question but if someone could answer that one two, by all means go for it. But my main question is am I being to rational or am I doing the right thing?