Doing my wifes cousin

so my wifes cousin has pretty much made it clear that she wants to have a threesome with us, that or just have sex with me, who really knows?...shes pretty hott, and my wife would do it (almost did last weekend)though i dont think she'll have to be drunk to do it lol...the only thing is that she has a bf whome she is cheating on, with at least 2 other guys, 3 if we every do it lol...so i dont find it bad having sex with her even though she has a bf...so, is having a threesome with your wifes cousin normal, considering her other men too?