Brother cheats on his fiancé for three years, family and I found out this weekend, and brother still wants to plan a stag do. How do I get out of the plans?

So my brother has always been the type to endlessly chase women. To the point it felt uncomfortable not just to us but to the women themselves. He's now been in a relationship with this admirable and impressive woman for 6 years, and who's been a part of our family and we all love her very much On fathers day, she came to the house a few minutes after my brother. A few uncomfortable hours because she wasn't her confident self, she was on her phone a lot, she was quiet and timid. My brother asked my dad if they could go upstairs to talk. That's when my dad got the news My brother was going on her phone, deleting incriminating pics of him with his cheater, and his fiancé called him out on it. He'd go home late every day at 10-11pm, he put a lock on his phone and he'd receive messages from his other lover like "I'm waiting for you, come over". This went on for 3 years and he knew the other lady in person I'm appalled, and I've never seen my parents this disappointed, especially since they knew he was always a liar and such His fiancé said she was going to think about whether the wedding goes on. Now my brother is sending WhatsApp group messages to us brothers and his friends, saying he's excited for the stag do. His friends are motivated to go and sound excited to celebrate his stag do in Oxford Circus. They probably don't know. How do I wriggle my way out of this?