Why not mother and daughter competing aware or unaware of the sexual interest from the other for this other man? A neighbor, a family's friend (younger or older). I don't find it so exaggerate.My personal taste is to see mother and daughter fucking with another man.
But I realise it's very difficult.
Firstly, because if you want to keep a consistent line, it is very difficult to create honestly acceptable dialogue. Otherwise you should exaggerate, but I have noticed that many people prefer almost the normality of a novel.
Nevertheless, seeing that you are already carrying the cross of unfinished projects, I will concentrate in finish this one before embarking in another one.