Should I confide in someone about my mom's infidelity?

I know my mom is cheating on my dad, and I believe it's a recent development. I don't want to tell my dad, as I don't feel it's my business, and I don't want to embarrass him; I love him too much. I thought about confronting my mom and telling her it's hurting me, but I'm not sure of that. The thing is, I need someone to talk to. I can't keep it bottled up. I'm graduating high school in a couple weeks, and I feel like this was a bad time to find out. Should I tell my sister, who is 22? Or should I find somebody that doesn't know me?