Actually ehentai defines it as any sex parent + any sex child(ren), but we don't have male/female tags here.

"A threesome with a mother and her daughter."

By the way, the literal translation of oyakodon is “parent-child bowl”, so it doesn’t HAVE to be a mother and daughter.

Technically oyskodon means mother and child. It's almost always a daughter, but I don't think it has to be?

A threesome with a mother and daughter simultaneously I think it should be more detail

Simple: "Mother-daughter threesome"

A threesome involving your partner's mother - UrbanDictionary So I would say, "3some with a mother and daughter pair".

agree with hulk hogan

Too vague, the correct description would be "group sex (usually ffm threesome) involving a mother and her daughter"