Trying to be a supportive sister but good lord!

I just needed to talk about this because everyone in my family is wanting to pretend this isn't happening. My brother has been dating this girl for 6 months and right from the start I knew there was something off about her. She just gave an eerie feeling. They introduced each other to family really early and me andy brother are close, but her and her brother are wayyy to close. She literally was changing in the living room in front of her brother. They would also male sexual jokes about each other. Just weird shit. My brother had told me he had something to tell me. It was that she was pregnant. Now my brother suffered a really bad injury when he was 14 and can not have children. So I was pretty shocked. He said "Well I guess I have some swimmers in there." I didn't say shit. Not my place. A couple weeks go by and he calls really upset saying she cheated on him. Was I surprised. Not really. Until he said with who. He said she told him she got really drunk and her and her brother slept together. I'm sitting here like.... HUH?! I told him he better fucking leave her. Then he got quiet and said " I mean, they were drunk, and it is just her half brother." I told him I'm gonna ask mom if she did drugs when she was pregnant with you because wtf. So he's staying with her. Even though she's most likely carrying her half brother's baby. And her and her brothers relationship hasn't changed at all. You would think if you regret hooking up with your brother you would stop being sexual with him. Joking or not! My brother was with me yesterday and he said "She hasn't been talking to me a lot the past few days. Is that weird. "

Yes Jeremy.... That is the one weird thing in this entire situation 🤦‍♀️🤦‍♀️🤦‍♀️