To think it’s inappropriate?

I don’t think it’s a double standard because you made your DD wait until 18 and DSD is only 16. Having said that, you know DSD will be bored shitless at yours but DH understandably wants to see her over Christmas so I guess having her boyfriend there might make it more bearable for all.

Realistically, you can’t have exactly the same rules for every child. They're in different circumstances, and the approach for each needs to reflect that.

It’s perfectly reasonable to be annoyed that the previous approach has been changed without discussion. I’m less convinced though that the only solution is backtracking on an invite thats already been offered.

The least bad solution may be to adjust tye details. Eg. One off because of the seasonal closures, separate rooms etc.

Let him sulk. He doesn't get to lay down the law where your kids are concerned and then backtrack when it's his own

If I were you OP I'd think really carefully about whether this is a hill you want to die on.
By all means let your H know you're not happy he didn't consult you. But pushing to have the plan overturned is going to have consequences. Your DSD is going to have to withdraw the invitation to her BF. She's likely going to be embarrassed and upset, and this will no doubt turn to resentment of you. She may decide not to visit at all and your H will miss out on spending time with his child. He is not going to like that, and will also blame you.
You may of course think all that fuss is better than letting the BF stay a few nights. Personally, I wouldn't. I'd establish ground rules, say it was a one off not to be repeated until she was 18, and that your H was responsible for entertaining them.

Offer him the spare room then.
Or offer him DSD's room and she can bunk in with her stepsister.

Sounds like your DH didn't mind being the disciplinarian with your DD but doesn't want to apply the same rules for his own.
It's going to cause a big problem now though because you'll be the wicked witch.
I hope he apologises to his DD and say he made a hasty decision and has to rescind his offer.

This

You made a choice as you didn't like someone. The OP hasn't met the man yet.

Do you have a spare room for him? Or an airbed in the living room. I realise this doesn’t stop them having sex but it means that she is more in control of what if anything is happening.

OP has no right to dictate if her SDD stays, as she is not her parent and that is up to her dad.

But she has every right to dictate wether a complete stranger stays.

My mums DP is a violent bully and I refuse to him in my home.

You can choose if you don’t want someone in your home.
At the very least, DH should have spoken to her before telling DD that she’s allowed to bring him.

This is a great suggestion, bring a girlfriend instead!

Well in that case your kids are also allowed g/bf sleepovers at 17. Which I personally think is reasonable.

If you don't want that for your kids then you have to say no to the BF. Could she bring a female friend instead?

Ofcourse I don’t. But, you don’t have to make it easy for her and by telling her your reasons why will reinforce the importance of being exceptionally careful as the consequences can be life-changing.

Presumably as you are well aware, because he doesn't not live with her and wants a relationship with her. One you appear adamant in not facilitating.

Well she’s 17 so she won’t be having him to stay at 16?

But you can tell your DD that the rules are different for her because she’s your daughter and it’s your choice.

Well in this case you can't make a joint decision, can you? Because what you want to allow your DSD to do, and what your husband wants to allow his DD to do, are two different things, so one will decide and the other have to accept the decision. My view is that in disagreements like this, the parent has the final word.

Her stepmum? Have you been actively involved in parenting her?

You are being very inflexible here. It sounds like she isn't welcome.

Do you think this stops her having sex? Not a chance does it

Coming from a household watching my teenage sister bring up a baby at the young age of 17, changing not just her life but everyone’s around her, and not for the best, I would be doing everything in my power NOT to allow this behaviour where possible, so especially NOT under my roof.

A 17 year old needs strict parents / guidance where boyfriends are concerned.

Presumably it was a fluid conversation with his daughter that he didn’t feel he needed to ask your permission for seeing as it has been agreed previously that boyfriends can stay over.

I really think you need to be flexible on this. It sounds like it’s the difference between her coming or not coming. You chose to live rurally, this is one of the negatives of that.

He's out of order not asking/discussing it with you first. If I were you, I'd say a firm 'no', and make it clear that this is the primary reason. Perhaps he'll think in future to make joint decisions on this kind of stuff.