Cheating dad

I found out my dad is cheating on my step mom and it's really sad and I hate going over their because I feel horrible for not telling her but I grew up with divorced parents and I wouldn't want my sisters growing up the same way. My dad tells them I don't go over because I think I'm better then them which is ridiculous. Sometimes I cry because it's either I let them think what my dad tells them or just tell them the real reason why I don't go. I hope one day when they find out they understand why I didn't do it.