It's a cest story, it's just not overt, it's pretty obvious. It's just been de-cested for Patreon. The story would only need to remove a couple of lines and change a couple more and it's one at it's core. Annie would be a cousin, and Penny and Dahlia are the sister's with Nancy being the mom. The writing in the story even implies it to me. Me, or any man I've ever met has ever had reservations about seeing, or wanting to see, a woman naked that isn't related to them.
Remove the line about MC's mom leaving, Dad divorces Nancy and moves to Europe, Dahlia an MC become twins basically, I think they are the same age, or they are just born close together, it happens.
Nothing about the way the story is written says it isn't one to me, to me the most of the writing even implies it, or I could just be a perve.

And if you look really close, you can see what Nancy is searching for on the web when the MC catches her.
Either way, the story is good.