Well, in my experience that will depend a lot on the kind of reltionship between the mother and her daughter have. There are also several games that I have seen where such type of throuple were the end of the game. In real life, I personally have seen it only once and from what I could see it was working quite well, but that may be the exception rather than the rule. It did have some similarities with the scenario in Lovephobia. Dad mostly absent, Mom rather needy so many people thought that it was a bad porn movie but alas they went forward. So in summary it all depends on the people involved.