This is a pretty common mistake Devs make with incest games. Holding Mum/grandma up as some kind of holy grail, like sex with them is the be all end all. Sex with them is just the beginning, or at least it should be. The start of a new relationship and taking things to the next level, leaves all kinds of possibilities to explore.

The quest for anal, the quest for pregnancy/starting new family, the quest for harem, lots of fun stuff that could happen after.
Sadly, most games end there or shortly thereafter. Makes me sad.