I think my dad is having an affair? what do I do?

[Emily]

I was looking for a picture in my dad's phone and I noticed a lot of pictures of this lady. they looked like a picture she took without anyone knowing like she was in public or something but then there were also a few pictures of boobs. she's the same complexion as my mom so they might be her boobs but tbh they looked too small and also why would my mom be sending my dad nudes if they live together? anyway I got curious so I looked at his Facebook and I found his messages to this woman. it was all pretty normal stuff but she kept calling him sweetheart. I know I'm not related to her in any way so she's not like his aunt or something. he didn't know I was looking at his pictures otherwise he probably would've stopped me. he's always very protective of his phone. he always has it with him and never lets other people use it. also his apple account sends every message he gets to his iPad and he takes his iPad with him everywhere. at first I thought he was just weird but now I'm realizing it might be because he doesn't want anyone to accidentally see a message from her? I know it's not much but I'm really scared. I'm the youngest of 3 children with the oldest in collage. I'm still in high school. what should I do? do I tell my mom? how could I find out more???