This might not happen due to Collective Shout, a christian organization heavily lobbying against porn on the internet.
They already influenced visa and mastercard to enforce more draconian rules involving adult content sales and iirc a few of the black listed things include family loving of any kind. Including step family and adopted family not directly related to you.
As a result, the game will get pulled from patreon if it includes Marin and grandpa so I imagine Inatari will avoid that.
Its possible to release it as a separate dlc/free add-on content but still risky to lose your entire income stream with a permaban without room for recourse so I wouldn't hold my breath.
A shame because I also wanted to see that pairing.
In theory, it's still possible. On Patreon, there are still several games that show relationships between step families and adopted families. However, from what I've researched, it depends on many things.
The first is basically chance. It seems that Patreon's moderation isn't the best, and unless you're unlucky enough to have a moderator come across your content or receive massive reports about certain content, they don't do reviews or anything.
The second factor, if I understood correctly, depends on how the story is constructed and the development of the characters or something like that. I didn't quite understand the explanation a developer gave me on that subject.
Maybe Inatari can find a way to bring Marin and the grandfather closer together. There's already a little dialogue between them where the grandfather admires Marin's body and Marin sees the grandfather with an erection. Anyway, we'll just have to wait and see because, as I said, Patreon is often random, since there's still content that could be considered banned on their platform, haha