Young to mid-teen daughters have a certain advantage: you can cast the mother as fairly young and still attractive herself, say, mid-thirties. That makes both of them sexually desirable.

I think these stories fall into two main categories. One is where both mother and daughter are victims of sexual violence This is pretty straightforward: rape, fuck, and abuse both of them. You can have one "give herself" to protect the other, then make the other one a victim too.

The second common motif is where the mother pimps or pro-offers the daughter for some reason. A psychologically valid one would be the abusive step-father figure: Mom overlooks, consciously allows, or even pro-offers the daughter to keep her husband satisfied. As a matter of fact, this situation is not unknown to the psychiatric literature. Lots of women have suffered incestuous-style abuse from stepfathers (or mother's boyfriends) which the mother was in one degree or another aware of!

(It really messes up their lives, in case you ever think of doing it for real.)

But for rape and bdsm fantasy, well, this can be pretty hot stuff!