Most text based games tend to be of wider scope.
From the ones I play you have TiTS, CoC, Wanton Cove, Academy of Fetishes, Fight Club, Lillith's Throne, Accidental Woman, Choker Change and things like that....
That's the kind of stuff you get in text based games. They cover stuff you can't find in other games.
Mom/son stuff is usually a bit too tame for them and is found everywhere so finding a text based game that focuses on that won't be easy.
Aside from Master of the House that has a bit of incest I highly doubt you'll find one that is just mother and son. I don't know of any anyway.