not an issue of stats being off. its a matter of it says "Requires: positive incest opinion"
There are plenty of other options.
1st pic attached is a screenshot of my old save. She dislikes Incest in that one and there are plenty of other options. I did have anal sex with her in this save, though, because that's required to pass the first dialogue that you can see on the 2nd screenshot.
2nd pic is a screen of a fresh save with her Love and Sluttiness maxed out with cheats. You can see that maxing out Obedience would also let you make her your girlfriend.
As for the 3rd pic, it's the Opinions menu that allows you to cheat their Likes/Dislikes to whatever you want. Set Incest to Likes/Loves and Bob's your uncle.
Also, all of that said, convincing your family members to become your girlfriends will switch their family specific Flirt dialogue to the generic lovey-dovey version. I, personally, found that a bit of a downgrade.