Soft YTA because you are a teenager and teenagers are supposedto be selfish and usually lack empathy. Let me give you a little perspective you said you noticed that throughout your childhood your father wasn't really present for you family. I am sorry you had absent father. You noticed he also withdrawn from your mother probably because he didn't know how to step up and put his family first. Your father has been selfish way before he started cheating. If he had any common sense he would not have cheated with with a family friends daughter that he was too old for and had have known it would get back to his wife and ruin more than just his marriage, family and friendships too. I also have know idea why you mention your parents 7 year age gap probably being the problem for their connecting when your idiot father screwed his girlfriend someone 12 years younger? Doesn't make sense unless you heard him say something about it if he did that is BS. Go back to you mom and say your sorry for not being the empathetic if you do value your relationship. You can tell her you felt you knew he would cheat because he has always been selfish and didn't put family first. Tell her you are sorry for what he put her through. She got the best end of it really because she is no longer with a immoral,immature loser and you Dad found someone at his level another immoral, immature loser. The whole town knows you father cheated on your mom with one of her trusted friends daughter. She feels embarrassed however I would tell your mom she shouldn't be becauseshe did nothing wrong. The ones that should be embarrassed is Daddy the cheater and his girlfriend. They both ruined that friendship by being horny selfish idiots.You said your Dad girlfriend is nice explain to me how she is a nice person? I can only think being the mother of your baby brother but everything else she done doesn'tseem nice. She slept with a married man that had a family. She slept with her moms friend husband, they blew up a marriage and a family. What did she and your Dad do when you were having issues at school because of the affair and divorce. Did they offer support or was that your mom? Did they offer to take any burden of your mom financial, taking you more while she was struggling? Did you Dad or her offer to pay for you and you mom to go any type thearpy? Or did they just continue they relationship out in the public now knowing you and your mo were struggling? From what you posted you Dad has put a lot of effort in your relationship with family before affair and now he is putting in some kind of work with new girlfriend that you justify because think he changed. Newsflash he hasn't as soon new relationship fades and your brother gets a little older and harder to take care he will do samething to the girlfriend. Thats why I think you mother is better off because you Dad will never change. Doesn't mean she needs to every forgive him or girlfriend they don't deserve to be forgiven even if there is a new baby. I think you might really love having a baby brother and thats great sounds like the only positive thing to come from this disaster.