Well, it's because she could be avoided and therefore not be considered as MC's sister, so that's an obvious reason why she couldn't be in the "Magnificent 7". Same for Tanya. But at this point like I already said, we shouldn't care about that. If you love the character that much, just think of her as a main LI. That's what I do with Tanya too.
Tanya has different circumstances. She's either in the game or she's not. If she's in the game she's pregnant.
Tanya is kept as a kind half character, she's kept on the property isolated from all other character other than the core family.
Sophia you could start her path or not, but she's in the game regardless. She's Cat's close friend regardless, so far as I'm aware.
I'd have had no problems with Sophia being tangential to the Core Family like Fiona and her family are, if she wasn't made our half sister. If she was just some random girl who our cousin is friends with, she wouldn't have any special circumstances.
Having her be our sister, and both us and her knowing, and then for her to be effectively cast aside from the family is just wrong.