I think we differ at the point where I think while I do think the daughter is sinning, she is going out of her way to not bring it into the mothers house. She isn’t having sex under mom’s roof. She even volunteered to make sure the younger siblings didn’t know she is sleeping in the same room with the boyfriend on vacation. I think she is doing her best to honor her mother. According to her faith, should she making other choices, yes but how is her being a good example and willingly keeping her not positive behaviors from minors a bad thing?
I don’t think this is a matter of what the daughter has done or is currently doing. It’s a matter of what the daughter MIGHT do when she returns.
After returning from Europe, (assuming she’s having sexual relations) she might want to spend the night at her boyfriend’s house. The younger children might not pick up on one “slumber party” a month, on a Friday or Saturday night, but they will pick up on it when it’s often or on a “school night”
Or when she goes out and comes back the next morning wearing the same clothes.
This issue here is the sin of scandal. If the daughter is going to keep her sexual sins (again assuming a sexual relationship) hidden from her siblings, then there is less of a problem.
But if it becomes scandalous, then it’s important to remove the scandal.
Finally, we 21st Century folks must realize what the sin of scandal is. If my actions lead someone to believe I’m sinning, and they use my supposed example as a reason to justify in their heads that their actions are OK, I commit the sin of scandal. Even if I’m not guilty of the sin.
For example: If I was single and living with a girl in a studio apartment and we remained chaste; I would still be committing scandal because everyone would assume I was not chaste and my younger siblings would want to follow in my footsteps.
Which by the way happened to me… my wife and I lived together in a studio apartment with one bed… while we were NOT chaste, we never spoke about sex. But naturally, everyone assumed we had sex and I strongly believe that our bad example led my younger siblings, cousins & mother-in-law to co-habitation and sexual sin.