I'm almost certain my dad is cheating on my mom. What do I do?

I'm 17 years old and live with my parents. As a preface, I went to a hotel on vacation with my parents about a year ago and my mom freaked out when his ex (separated for twenty years and with my half brother to show for it) sent him a text at 11:00 PM staying something along the lines of "good night my sweet prince".

He didn't own up to it and claimed that she was mentally unstable and suicidal, and he had helped her out. In my opinion, he was extremely unapologetic, even when my mother was bursting out to tears. After I had enough of their arguing in the hotel room, I did too. Mom realized what their arguing and pretty heavy words had done and tried to console me. We eventually went to sleep in the same room that night and from that point on, coupled with his weak response the day after of "I told her to stop because my wife and kid didn't like what she said", I felt like my father had severed a bit of trust that I had for him. From then on, I still remained skeptical but gave him the benefit of the doubt. He said he blocked her number and would stop interacting with her.

Fast forward to six months later, I noticed that he got a new polo for Christmas but don't know where from. I asked him about it and I don't quite remember what he said, but it prompted me to eventually take his phone and scroll through some of his texts. I know that it was a massive invasion of privacy, but I was just too curious. Among what I found were two entries for my brothers contact name which seemed odd, and the contact name of "Jerry" for a conversation including a picture of the shirt. Plugging in the number to an app that gives yo names on phone numbers, I saw that he got the shirt from the daughter of his ex (who said "thanks dad" in the text about the shirt).They're not related so I became a bit more suspicious. I know that they used to spend time together when my brother still lived with his mom, but I didn't know that they were still close. My dad comes home late from "work" every day except on Sundays, which he doesn't work on. He owns his own business so he works on his own time. Always has come home late - for all of my life. Surely he would have time to do other things besides work. I felt like he might be spending more time than I would've thought with his ex because of this (her daughters live with her).

Fast forward to like one month ago. My dad asks me if I could make my social media accounts private because apparently his ex messaged him asking how he could take his son to a high-end restaurant for his birthday (I posted a picture on Instagram in what was actually little better and no more expensive than a Cheesecake Factory) while his other son (her son) is being apparently neglected (completely untrue). For perspective, my brother is 22 with a daughter and wife. Why she felt he needed to be dependent was beyond me. Anyways, asked him why she was even providing her input in the first place and told him he shouldn't care what she has to say and should ignore her. Seemed odd to me seeing as he had supposedly "blocked" her.

Fast forward to today. My dad asks for my help with some home automation which required me to use his phone. He always deletes his call history and text history which always made me suspicious but I never found anything. I retreated to my room and instantly started digging in. He always deletes his texts which always made me suspicious, though I never found anything to pin on him. I checked his text messages, and saw his two different contacts for "my brother" were both recently messaged. I checked one of them and the name led back to my brother. I checked the number for the other, and well, it led to his ex. Welcome to the 21st fucking century, dad. In the conversation to his ex, only two messages were present, one of her saying she left her house or something along those and his reply saying "Ok love you". Took every fiber in my being to not call him out at that point. I simply held back my tears and gave the phone back to him, saying I fixed the issue.

I have no idea what to fucking do. I could give less than a flying fuck about his ex, as her and her family can't even get my name right. I know it's none of my business too, but I still live in this house and feel like I need to say something. Sometimes, my parents are really fucking annoying, but I still love them both. My dad has provided for me monetarily, but to be honest, my mom has been here for me most of my life and though I've recently been tired of arguments with her, I'm on her side right now. I feel really bad for my mom and am extremely worried about the implications of this. What should I do?

Edit: Just remembered that it's his birthday tomorrow. The very first incident a year ago happened on my mom's birthday. Irony, huh?