My brother cheated on his girlfriend with a minor

So for some more idea of what were getting into brother is a senior in HS about to literally graduate in 3 months and his side piece is barely out of middle school freshman.

During our usual monthly gatherings at my home where my fiancé, my brother, his girlfriend, and I converge, a fascinating situation unfolded before our eyes involving my brother and his exceptionally close female companion. Despite being in a committed relationship with his girlfriend, my brother seemed to gravitate more towards this friend, stirring a sense of intrigue amongst us. As we all sat together in one of our recent get-togethers, we couldn’t help but notice the unspoken tension and uneven distribution of attention within the trio. Sensing the palpable imbalance, we decided to cautiously broach the subject during a subsequent hangout. When we delicately probed into the nature of their friendship, my brother’s girlfriend brushed off any suspicions, labeling the friend’s actions as mere over-attached behavior. While we attempted to accept this explanation at face value, the underlying complexities continued to nag at our thoughts. The disparity in interactions and nuances of their connections left us with lingering uncertainties about the authenticity of their relationships. Upon our latest rendezvous, a tangible divide had formed between my brother and his girlfriend, intensifying the enigma surrounding their intricate dynamic. As my fiancé and brother retreated to the shed for some male bonding activities, I found myself engrossed in a heart-to-heart conversation with his girlfriend indoors, where she candidly disclosed the growing strains in her relationship with my brother, shedding light on the deep-rooted issues that had started to surface. They were considering breaking up in August because they were too different people who had different views and it wasn’t her that initiated the breakup, he did and didn’t fight for the relationship. He just agreed. After we had our conversation the boys came inside and we tried watching a movie, then she broke down and asked why, is he doing all this, why won’t he fight or do anything for the relationship. I won’t lie i kind of instigated the drama and said it was probably that stupid girl of his. She was considering it and when my brother was in the bathroom talking with my Fiance. She noticed his phone and knowing his password we went outside and saw very incriminating messages between the two. It was disgusting, he was calling her “baby” saying he “couldn’t wait to end it off” “once she was gone that LITTLE GIRL, would be all his”. Knowing what we did i couldn’t stop her from confronting him and honestly it got bad. My brother got violent and starting going after her scratching, biting, bruising. My fiance got in between them and my brother bit him so hard it left a bruise i literally had to pry his teeth off. We decided to have his girlfriend sleep in our bed and brother on the couch and by morning we would send them home separated, his girlfriend decided to get everything out of his room and if our parents asks she’s gonna go full guns blazing and tell everything. How he cheated first of all and how the person he’s cheating with is a CHILD. Honestly I’m pissed, definitely not as much as her but it’s just disgusting to know what I know and I have no idea what to do.