I'm not sure if this is the right tag or not

I (19F) have a brother (17M) who's making a terrible choice and I really need some advice.

He has been dating this girl (17F) since December and things were going well until at the beginning of January they had an positive pregnancy test and were expecting in September. We were shocked but were willing to help them as much as we can. I was so excited to be an Aunt.

They went to the doctor for an ultrasound in February but there wasn't a baby in there and turned out she wasn't pregnant after all. My family and her family were.getting used to the idea of a new baby just to hear that it wasn't even happening in the first place. We bought her around $300 of prenatal vitamins, baby books, and pregnancy clothing.

I thought something was kinda weird with this cause something didn't add up with the timing and felt off somehow. The test they showed us looked kinda strange so I did some research cause I never seen those brand of tests before and saw positive pregnancy tests on Etsy that looked exactly like the one she showed my brother. Than it all made sense: she tried to baby trap him.

My brother showed my family this and they were completely devastated that she did that to him. To make matters worse he tells me that she has been cheating on him with 5 different guys (one of the guys showed him screanshots of her texting other guys). Honestly it made sense because he said that she had "strict parents who won't let her go out" on some days even though they were fine with her going out with him normally. She specifically told him on Valentine's day that her parents won't let her see him even though they normally do.

He broke up with her for all of this until I heard from him that he got back with her in early March and not only that but he PROPOSED to her. He said that he's keeping this a secret from our family and that on his 18th birthday in August he's going to tell them he's engaged and moving out and living with her and getting married in January 2026 (Btw he works part time at six flags and she doesn't have any job, they literally could not afford to live in even a tiny apartment cause he also overspends his money on shoes and stupid stuff instead of saving).

I told him this is a horrible idea and could potentially ruin his life since she's cheated on him before and also tried to baby trap him. But he says that I'm just "jealous" that he got engaged before me and that he's moving faster in life than me. (Me and my BF (18M) started dating in highschool and are only getting married once we are financially stable and mature enough for marriage).

I told him that I'm not jealous I'm just trying to stop him from making a terrible choice that could ruin his life and that he shouldn't rush into something like marriage especially with someone who hurt him. He called me a jerk and an "opp" for not supporting this and for hating her for everything she did to him.

He claims that she "changed" and that he forgives her but keep in mind that they just started dating a first met December 2024, had the baby news in January 2025, the "no baby after all" news in February 2025, and got engaged in March 2025.

He's going to end up divorced or living a terrible life cause he refuses to listen to anyone who tells him not to do this and thinks we're just hating on him. I don't know what to do. He won't listen to any adult who tells him not to do this and thinks we are all assholes for not supporting this. So AITAH For wanting my brother to break up with her and call the wedding off? And WIBTAH if I just let it happen and not do anything about it?

Edit: I forgot to add this but while they were together they constantly had me do favors for them like give them money for things and even pretend to be a "friend of hers" by being in the car driving while he's in the trunk, drive up to her house so her parents see me and not him, pick her up, drive far enough so they can't see us, park and have him switch to the driver seat and me in the back seat, and he drives me home so they could do stupid dates. They basically used me so they could sneak off and basically did this for every date they went on. I never got paid for this either, the only payment i got from them was a giftcard only cause her brother called me ugly while pretending to be her friend, not for the millions of times they needed me to drive to her house and pretend to be someone else just so they can sneak off. And they still ask me for money constantly or if our mom has any. I always lie and tell him we don't have any just cause I'm sick of them using us. I'm not sure if that's an asshole thing to do.