I find it has issues with or without the incest. Anyway, if you use unren it should be easy to delve into the game files and activate incest.
I might end up doing it myself sooner or later, but like TeslaCoil below I'm still unsure whether the story makes more sense as incest or as a genuine tenant situation. But if you want it and need help, PM me and we'll see. (I'm also more than a bit curious about what would happen if I posted a self-made incest patch.)
Yes, an incest patch should do much more than just change the relationship names as there are a lot of inconsistencies on both routes.
I'm leaning toward incest at the moment, as there's some taboo not only with Priscilla (that can be explained as "he's my friend's special son, I can't take advantage of him!") but also with Luna, and I mean, who would have second thoughts about laying his sexy friend only because she used to rent her room from the same woman?