I think it shows great maturity from your daughter, she has obsviously thought it through and reflected on how it may affect her.
I am not in this situation so i m not qualified per say to comment, but I am a mother of 2 teens one of them being 16.
I would treat it the same way i treat any “demands” coming from my teens and especially the 16 yr old. Ie with common sense.
I would thank her for talking to me about the / her future. I would actively listen to her worries to decide if it is a fear of change, if it is something she s read about the wicked stepfather, something she s discussed with her father and / or her friends who could be in a similar situation.
I would then tell her that she is right. No one is trying to replace a father figure for her. And that whilst she has shown fabulous independance and reailience and that you trust her and her judgment ao far, the expectation is for her to respect the people living in the same household as her. It isnt about telling her what to do but about living in a community and respecting the people in that community. Great skills to learn now if she goes on to live in a shared house at uni. She ll be living with people she didnt know before.
I would also highlight the fact that this person is coming with his own wealth of experiences and that obviously you trust him since you want to marry jim and build a life together. Therefore she could be surprised that this person may be a benefit to her life too with different kind of knowledge, friends and experiences she could learn from and benefit from.
She could also develop a kind of special friendship with her two step siblings. Point is I wiuld encourage her to keep an open mind.
Maybe it is the time for her to develop a more grown up attitude and friendship towards new adults in her life. Seeking guidance from different viewpoints.
So whilst you would be doing the day to day “policing”, I would remind her that it takes a village to raise a child. And that a village isnt only family.
I would also remind her that she is still in her formative years.
On a practical level, why not setting aside regular chats in a neutral place to discuss and talk about the living situation so you can minimise the potential issues and solve them before it becomes a big deal amd bring resentments?
But I still believe more than ever now I am faced with teens, that they do still need boundaries and reminders that there is a whole world around them and of which they are not the centre
.
If you rephrase what she said in a positive adult manner, it is actually for both of you a fantastic opportunity to talk and to get even closer.
Good luck