My Father Is Dating Another Woman… Should I Tell My Mum?

I observed recently that my dad is cheating on my mum with this woman at work. I saw some texts on his phone exchanged with her saying really lovey dovey things, and I'm certain it means he's cheating.
I have no idea what to do. I cried a lot, but now I'm just trying to think things through. I desperately want to tell someone, but I have no idea who to turn to, and I'm worried that if my dad isn't cheating (I hope to God that's the case), he will hate me for thinking he is.

But I see no way that he can't be possible. I saw those texts, and they say things like "love you", so he must be cheating. He doesn't know that I've read the texts…
At the same time, I don't want to be the one who finds him out. It would completely break my mum's heart. She really loves him, and I don't know what it would do to my little sister. I just can't imagine our happy little family ever breaking apart…It makes me cry just to think of that prospect.
Why would he do that to us? I don't know what else to say, just, please help me. I'm really scared.