The thing is, they are sisters and they aren't supposed to do it, or at least that's what they believe.
The goddess said sisters should "treasure chastity", not that they should remain absolutely chaste. It was a purposefully vague wording.
Remember, in case of Lily, they having sex was sort of a mistake or more like, she couldn't hold it in and went ahead, taking Itsuki by herself. Of course they later kept on having sex because after the first time they couldn't go back (both Lily and Itsuki we're pent up).
In case of Talia, it happened mostly because she was seeking Itsuki more aggressively. She was curious about sex and, being more rebellious than the other sisters, she decided to go ahead and break the taboo, or rather, it wasn't that much of a taboo to her in the first place.
It's not that sex with the other sisters won't happen, it's just that they have to break the taboo in their minds first, to then proceed with it in reality.
I don't think Lupina will be that against it, but I'm pretty sure Rose will be a hard nut to crack.
We'll just have to wait, and see how things develop.