God GDB ...... I'm echoing Fenny here. Being a stepmum is hard enough without all the drug & drink issues.
I don't blame you for not wanting this girl as she is now (presumably nothing's changed then ?) in your life, and more importantly, in your children's lives.
Even leaving you aside - that's three (including the new one) young children who could potentially be at risk - either physically and/or emotionally from this girl. That sounds really harsh but I'm sure I don't need to tell you the truth is that someone - anyone - who is perpetually stoned and/or p*ssed cannot be trusted round young kids ..... whether or not any incident is intended. It's simply not safe, and could be frightening for them too.
Does DP not see this ? ...... I can understand him not wanting to lose his older daughter but he cannot jeopardise the safety of the younger - and therefore more vulnerable - children.
In the circumstances, especially considering how much effort you've put into trying to help this girl and get all involved to participate in a properly thought out plan of action (i.e. you've not, up until now, turned your back), I think DP would have no right whatsoever to expect you to continue to put up with her presence in your home. It's not you being spiteful, or selfish, or awkward, or anything else .... but you protecting very young children.
What has happened sinec you last posted ? Are DP and his ex "toeing the line" so far as the "agreed" plan of treatement is concerned ? ....
.... for example, you said before that she would not be allowed to stay if intoxicated, and that a residential home might be on the cards ?
I do understand DP's dilemma but sometimes you have to be cruel to be kind in life. If DP always provides a "cushion" when she behaves badly (for want of a better expression) she will have no incentive to sort herself out. If there are no negative repercussions to her unacceptable behaviour, why should she stop it ? Effectively, he is enabling her to continue behaving like this if he is always there as a crutch, no matter what.
Now I know she's only 15 and I can't begin to imagine how worried he must be, but at the same time there needs to be a structured approach to all of this so everyone involved knows where they stand, what is expected of them and what will happen if xyz is or isn't done. From your last post I got the impression that inconsistency was one of the problems.
I really think you must, for the other kids' sake, make your feeling clear to DP. Emphasise that you are happy to offer whatever practical help you can so long as the younger kids aren't at all involved. Emphasise how you hope she can sort her problems out and how you'd love to welcome her back to your home, hopefully sooner than later, but it simply isn't safe or right to do so in the current circumstances.
I'm afraid if he kicks off about this, "blaming" you or "resenting" you then that would speak volumes about his feelings for the other kids. Three of them cannot be put at risk for the sake of one.
Good luck .... I don't envy you one bit having to deal with this.