I don't know what to do next

My dad is most probably cheating on my mom, and the worst part is , only i know about this...

He cheated on her for 4 years..but eventually he ended his extra marital affair and they lived happily since then...but somedays back i saw on my dad's phone ..that she(his ex affair) still message him ..and he saved her number by the name 'honey'..

The audacity! I got very anxious and don't know how to deal with this feeling...if I'll tell my mom.. she'll go mad again...(Last time she became suicidal and left home)

I don't know what'll happen this time...

I can't tell my elder sister because she just got married and enjoying her honeymoon period... I don't want to take away that happiness from her..

I'm too afraid to talk to my dad on that matter because he has been the best father...he gave me his best..learnt parenting for me..took classes for academic so that i don't fall behind and he can teach me...but he is the worst husband to my mom..

I don't know what should i do next...

I meddled in between of them before too...but eventually i got hurt and my mom and my dad resolved their dispute... I don't want to face this situation again