Two gentlemen become Wiener Cousins when both are Wiener Brothers with the same third gentleman.
To further explain, if Jeeves and Jay have had both made love to the same woman, thus becoming Wiener Brothers, and Jay and Aaron have both made love to a second woman, creating a second Wiener Brothers relationship, then Aaron & Jeeves are Wiener Cousins through their shared relationship with Jay.
Jeeves: So I porked Chloe the other day
Jay: Yay! That means you're Wiener Brothers with Aaron! And he & I are Wiener Brothers, so now you and I are Wiener Cousins!
Jeeves: Honey I'm so happyyy!
Jay: Me too, buddy. Me too. Now all you need to do is have sex with your mum and you & I will be Wiener Brothers too!