What? Why? Heeeeeelp??

Okay so one of my friends is going out with my brother
which is a bit weird
but really cuute lol

anyway atm we have a spanish exchange going on at our school
so some of the people with exchange students went bowling tonight

Then i heard that my friend cheated on my brother with a spanish guy...
but ive been sworn to secrecy

and i want him to know
but i dont want to tell him
and i dont want him to be upset
but he really likes her
even though theyve only been going out for a week

also im not supposed to know
so even though im pissed off with her i have to pretend i know nothing
and its even harder cause i sit next to her in maths
and im going to town with her tomorrow
and others ..so its not as if i dont have anyone else to talk to
but my brother will be there

what do i do?
do i tell him or not?
do i talk to her or not?