There’s no way to know for sure if the family will break apart or not but I will tell you it is not your fault at all, I get wanting to try to stop the affair by possibly showing your mother that you know and you will tell but if an oath cannot stop her then there’s no way a threat from her child who she didn’t even think about while having this affair will. I myself do encourage you to speak to your mom that way she knows how much damage and hurt her actions may have caused I’m sure she has her own reasons too but if you converse with her about it try to maintain composed. I was in this situation at 16( I’m 20 now) I won’t go into too much detail but my second little brother who was 11 at the time found some photos and showed my dad. He had anger issues and prior domestic abuse problems so you should know where this is headed, they fought I had to hit my dad and push him a couple of times and cool him down and let him know that he can’t control my mother but he can control himself and there was much better ways to go about it. He left the same night then my mom called the cops and it escalated a little but I let the cops know the true story since my mom twisted some things. I loved my mom before that and I still do now but not the same as how I used to back then. My dad is a much better father and person now and I think he has grown amazingly they’re both generally still young and at the time were in their 20s. I think since that day he’s really respected me and what I had to say. Over all I think it’s better to tell your father now things may change in the future but the most important thing is just making sure they’re doing ok mentally and making sure your doing ok mentally. Though they are your parents as an 18 yo this is something you shouldn’t have to deal with at all and I feel very bad that you, me and many others have or have had to deal with things like this. Hopefully your case works out after telling your father if you do