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.
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!