Fauxcest

Fauxcest is the most commonly used term to define the practice of acting as and addressing one's partner during a scene as if they were a family member - "Dad"; "Mom"; "Big brother" - during sex (predominantly BDSM, as it most often accompanies various flavours of CNC).
It's way more common than one may think.

Me and my boyfriend tried doing a Fauxcest scene last night - I didn't think he would've liked being called "big brother" all that much...