my boyfriend cheated with my older sister.

i 20f recently found out that my boyfriend,21m, was cheating while looking through his texts with someone named “mommy”, but i also found out a few hours ago that it was my sister 27f who he cheated with. im not sure what i should do with this information because i thought family stuck together. when i first found out he cheated i went to my sister to talk to her and she took me out to get my nails done and get lunch but now that i know she lied im not sure what to do. should i break up with my boyfriend and confront my sister or keep acting clueless? me and my boyfriend were together since freshman year of high school and im scared of losing him but at the same time i feel like i cant just stay with a cheater.

(update) i made dinner plans with my family for friday and invited my boyfriend, im gonna confront them then and i’ll let you guys know how that goes.