My sister in law is cheating on my brother!!? What should i do??!?

ok so my brother recently enlisted in the marines hes been away for almost 6 months but we have seen him for 10 days after his bootcamp. he got married that same week to my sister in law. but i recently found out shes cheating on him. i know this because i found a note that she wrote to a guy that said that she would love to meet up with him and stuff. The worst part is that the guy is my brothers best friend! He was his recruiter for his marine enlistment. I am certain their cheating because i read her facebook messages sent to him and they talked about how fun their hooking up was and that they should meet up again sometime. and also how she sneaks him into our house at night! I know i shouldnt be snooping around her stuff but i had to i love my brother and i dont want him to get hurt! Another thing is that me and my sister in laws relationship was really good up until i found out about this. She doesnt know i know but she thinks im acting really strange. Also i printed the messages so i have proof if my brother doesnt believe me or if she denies it. They have a daughter so im not planning on telling him soon because it will ruin their marriage and it hasnt even been a month since they got married. and i think shes a goldigger beacuse she doesnt work and lives at my moms house also shes not legal here so i think she wants my brother to make her a resident of the US and after that shell probably file for divorce! I dont want to ume this but i seriously dont know what to think now! I just told my sister because i needed to tell someone i literally couldnt sleep just thinking about this! even thought i know its none of my business. Like i said before i really care about my brother and i cant stand the thought of him being cheated on while hes risking his life to have a better future with his family. i would greatly appreciate it if anyone can give me good advice. like if i should tell my brother, or not tell him or wait till he gets home again and tell him then? i know i have to tell him someday because im pretty sure my sister in law will never tell him she would just live with the guilt. so please should i tell him??????