To ban SD from my en suite?!

I really don’t see the problem with her using your en-suite when you’re not there. That wouldn't worry me at all (assuming she doesn’t leave it in a mess). It’s just a room.

She shouldn’t really use your products, though, and certainly shouldn’t wear your dressing-gown.

You are a family ! I wouldn't dream of telling my children or stepchildren that they couldn't come into my bedroom. My children when they got to teens and early 20s seemed to find the need to talk to me the minute i got into bed, the boys would lie on the bottom of the bed and yap for ages, my daughter would get into bed to chat to me and fall asleep, in fact even now when i visit her or she visits me she still comes to chat to me in bed.
I would draw the line at any of them using your products, dressing gown and slippers though ( not that my daughter would take any notice)

What does your partner have to say about this ... a crucial piece of information imo...

I think the issue is that OP has said the en suite is not to be used by the children. Rather than discuss that with her and negotiate a compromise, her SD has helped herself to the space, products and clothing. It's an invasion of privacy. If her dad is aware this has been happening he should also have raised it.

Sorry but NO !! Would not be at all happy with this from my DC let alone SDC

Using the en-suite when I’m not there wouldn’t bother me, but I’d be annoyed at anyone using my stuff and especially wearing gown/slippers. Does she wash them afterwards or have you just been unknowingly wearing it after her?

I get it but I would alse be quite flattered that she wanted to use my stuff and would use it as a bonding thing. My niece loves a super long shower and taking ages to get ready and I happily give her my bathroom products and dressing room when she is here so she can enjoy herself it’s a nice thing ?

My suggestion: go into your SD's room while she's there and help yourself to her stuff. Makeup, beauty products, clothes, accessories, shoes, whatever you fancy. Sharing goes both ways after all!

Her behaviour is very strange and invasive, but maybe doing the same thing to her (or at least pretending to) will open her eyes to what a massive boundary violation it is.

I'm curious btw, what did you say to her when this happened, and what was her reaction?

I think I’m probably going against the crowd but I don’t think I would be bothered anyone using my en-suite. But I don’t have stepchildren, I have 3 kids (2 boys, 1 girl) and everyone uses whatever bathroom is most convenient at the time. The only thing that would upset me would be using my stuff without asking so I would definitely have a word about that!

This
Keep this in perspective OP.
Just reinforce your requisite re use of ensuite.
Pick your battles

Finding these responses really weird and feeling grateful that I didn’t grow up with such an uptight mother!

I’m really surprised by this. I wouldn’t care if my kids used my bathroom or my stuff or if my DD wore my clothes. It’s their home and they are my children, I literally grew and birthed them. They are teens and they are messy but my home is their home. Surely!

Or is this a step child thing?

My teenage daughter knows to ask permission to use my en-suite. I have a far superior tub. It doesn’t harm me to let her use it on occasion, but she does check to make sure monopolizing the area for an hour won’t be a problem.

I think pick your battles. You weren't using your ensuite. So a bit dog in the manger to be mad about that providing it's cleaned for your return. But your personal items she should not be using. However you need to say this firmly but without rage. Otherwise you could do some serious relational damage over what is essentially a minor issue in the bigger scheme of things.

I understand your rage though.

So what happened? Was she embarrassed?

I don’t even let DH use the toilet in our en-suite, YANBU

What was her explanation when you asked her why she used your stuff?
I would tell her that if she did it again, especially without asking, you'd make her pay to replace you creams/lotions she's used .

Christ, no! Grown women are allowed to have their own stuff. We don't have to tolerate our kids or step kids pilfering them whenever we're out of the house.

Get yourself a lock as clearly your DH won't enforce your boundaries and your SD takes the piss.

She can ask for a nice bathrobe and toiletries for her but if she's that desperate for them.

Is there a bath in the kids’ bathroom though?

I wouldn’t mind her using the room but using and especially wearing my stuff isn’t great. Does she have her own dressing gown and decent products? As she thought you were away and was having a pamper your DH has been allowing this I think you need to be kind. Are your products really expensive or something? I have never bought toiletries I wouldn’t want my teen or his gf to use. If. Something was a gift or a bit special and I wanted to keep it for me I would keep it out of the bathroom. Both bathrooms are available to all of us as one has a bath and one has a (really good) shower.

Has she been directly told not to use the en-suite? Did you bollock her? I’d be really pissed off. That’s your little piece of privacy and she’s invaded it. Not on. Assuming she has funds-allowance etc, she can get her own products. I have expensive curly hair stuff, I don’t want to run out because someone else is using it.

Don’t we teach our children to share?!

I can’t imagine forbidding my children from having a bath in my en suite and using a bit of bubble bath and shampoo and cream if they wanted as a little treat!

This. Take a chill pill, it's family and sharing is part and parcel of that surely.

Does she have her own dressing gown & slippers?

could she not have these so basically borrowing yours?

You know what to get her for Christmas at least!