wife wants a threesome with my brother's wife. no.

title is the whole thing basically. she brought it up twice. first time joking. second time not joking. my brother's wife is allegedly into it apparently they talked at the cousin's wedding in Manchester last month, I was outside smoking with the uncles. brother doesn't know. that's the part. she wants to do it without telling him. says he'd be 'weird' about it. yeah he would be, he's my brother, he lives forty minutes away, we have Sunday lunches. I'm 41, software, two kids. wife is 38. been doing the swinger thing two years, never in our circle, that was the agreed line. now she wants to cross it and lie to my brother specifically. am I the problem for saying no. she's pissed at me. won't talk about anything else. would you do it.