My mother is cheating my dad very badly what shud i do?

a month ai alwyz thought my mom was devoted.....i m 13year old boy nd ve a younger sis who s 9...lat motnh my father went to official trip one day my mother sent me nd my sis to pur aunts house she used 2 do this evryday...oneday i was really suspicious so instead of going to aunts house i returned nd saw my mother,who is 35 cheating on my dad with a young 25 or 26year old man....she was desperately kissing him nd striping font of him nd he was enjjoying it too ......i think they must ve done this everyday.....wht shud i doi shud i tel abt this to my father...but they will get divorced! i m scared!