I’m very tensed & confused

[Me1]

Assalam o Alekum
So to start off this, im the youngest(13) in my house and I just have one elder brother who is 3 years elder than me. Me being his sister. I had a feeling he has something going on a with a girl in our school we go to the same school and so does that girl in his grade. I was confused but later minded my own business, soon then just like yesterday I just picked up his phone and checked his WhatsApp he was gone for prayer outside I know it was his amanah and I should not check other peoples privacy but I just had to do it and I caught him red handed he was after all chatting with this girl I started shaking and my hands were sweating like they are now. I cried a bit and recorded the messages in my iPad just like that they were not talking about work they were taking about like “hey U saw my goal that day in PE?” “oh yes that was great I never knew you could play so well and similar msgs and my bro congratulating her for stuff and so was her I was sure it was her he didn’t save her name but like defaulted from WhatsApp it was her ik. Not only that I was sad but angry at the same time, I never knew I would see this kind of day in my life but now I have. Now hear me out I haven’t told my mom as of now cuz if she listens to this she will be reactive emotional angry and I’m scared to tell so quickly also my brother is doing hifz he gets topper marks in school he’s excellent at football he goes to all prayers he has very good knowledge of islam he is obedient respectful has good friends in islam alhamdullilah we are quite rigid my mom doesn’t even allow us to celebrate birthdays as it’s like not a celebration done by Muslims but still some people do it which isn’t Haram but is not recommended. She carried us for 9 months I literally was crying I never knew he would do this he is really good at school and in islam Mashallah not to approach about this to my mom I just don’t know how to do it, should I directly tell her? Or my brother has a very close friend and I know him very well too should I meet him in school and tell him pls explain my bro and share my sorrow with him cuz I just don’t know who to share it and I don’t have a elder sister or so and it’s my responsibility to stop my bro from doing this even though I’m younger allah could ask me on the day of judgement that when I knew why did I ignore? Why didn’t I stop him? And I can’t just directly tell my bro it’s embarrassing we have a tradition too. pls can ya’ll give me some advice should I tell my bro’s friend so he can manage something with me or directly my mom who could faint literally or just do something or ignore! Jazzakallah Khair may Allah (swt) guide us all to the straight path ameen!