My mom is having an affair in her marriage

Hey ladies before y’all try to come for me please think about 1. If the gender roles were switched 2.if you were in this situation. So my mom has been married for 21 years to my step dad I didn’t know he wasn’t my biological dad until I was 16. And also have a lil sister. Growing up my dad was very aggressive in tone and was not emotionally available for us I’m assuming that’s how most african men are. Anyways he would always take us to church, pray for us, buy us clothes and stuff. He helped my mom get her green card to work and stuff. He’s responsible, paid the bills and sure we had food. As i got older I realized my mom was miserable in her marriage they NEVER took a trip together, he’s never done sweet gestures like buy flowers, cook for her, massage her feet etc. he’s a shopaholic so he’d buy her designer bags, shoes clothes etc. he just wasn’t adhering to her emotional needs until she threatened him for divorce last year. Now he bought her cake, flowers etc. take her out to eat etc. but my mom is currently on a trip with a ex she was with before my dad and i feel horrible. I just booked a room for her she thinks crazy and don’t know she’s cheating. I feel so bad because my dad don’t deserve that. If she dont want him then she need to leave. The man she’s cheating with also has a woman. This is messy. What should i do? I almost want to tell my dad. They are still married and living together it is wrong. I’m happy she’s happy but it’s not the way to go about it. I guess she waited until my lil sister was of age until she came out wanting a divorce. But I would’ve rather we live separate than to be in a household where my dad always yelling and not emotionally available. she paid her carnote alll by herself and he always wanna drive her car when she off. He’s very cheap and stingy. He NEVER gave us money until this year and i have to pay him back