Ah, yes. I always take my beliefs about sex from Queen Victoria. She was the Dr. Ruth of her day, after all.

perfectly socially acceptable since let's face it, most female family members have sex with each other anyway.

This is where I think he must have been warped by porn. Threesomes involving a guy with a mother/daughter or sister/sister pairing are frightfully common, even in the mainstream stuff. Of course, normal people know porn =/= reality. But some people are stupid.