I think the question of "When?" isn't as interesting as "What happens after?".
Like Nana starts off fairly proharem in terms of being fairly supportive of us given her as many great grandchildren as possible and us spreading love etc.
coming up to her finding out about all the incest she started getting increasingly possessive of MC, and the increased again after finding out.
So I'm wondering how her giving into the incest will change her behavior. She'll be in the harem, and that will take some getting used to, but like after that. I can't see her being a recruiter like Cat, Fiona, and Aunt Liz. And none of the others have been particularly possessive of MC like Nana has been in the past. Well, Mom has been, but not in the same way or to the same extent.
I do hope for and look forward to Nana saying she's sorry to MC and all the girls in the family for how she reacted when she found out. Poor Cat looked like she was afraid of being disowned