I'll try and keep this as brief as possible with all the relevant background.
DSD has a history of witholding contact with her dad whenever she doesn't get her own way. She won't have a conversation when she is wrong about anything and she refuses to accept any boundaries. She will never apologise for any bad behaviour, she'll just sulk for 6 months and not see us. OH is excellent at taking up the mantle and keeping me out of it.
A couple of weeks ago OH had a big ding dong with her about her social media account (she keeps uploading provocative photos and making her profile public) rather than agreeing that she was wrong, she refused to speak with OH for a couple of weeks and carried on. We had a deal with her months ago that if she did it again, she wouldnt get her allowance for that month. This was all very clear and understood.
At christmas she was horrible to everyone on xmas day. OH had begged her to come to see us, right at the last minute she said she would, once again leaving him treading on eggshells until she arrived at a moment's notice.
On xmas day once she'd arrived, she was rude to everyone. Telling us all she didn't have to be there, she'd only come for the presents and then disappearing to her room as soon as she'd opened them. OH caught her drinking from a bottle of vodka in the kitchen around 10, then when he went upstairs to tell her off she laid into him massively. (She's 16 btw) we've had to buy bottle locks for the house after a few instances of this before and stupidly we'd left this one bottle on the top in the other room.
She was telling him she didn't want to be there, that she felt left out (we'd done everything we could to welcome her and rise above the previous few weeks), that his life was tragic and he was an embarrassment to her, she shouted at him asking him what he was going to selfishly spend her allowance on the next month because he wasn't giving it to her.
The following day we had grandma in tears as she did the same routine. coming down from her room at 2pm to open her gifts then straight back upstairs.
We've tried several tactics this year, rewards for good behaviour, we redecorated her bedroom at great expense to make it feel more welcoming at ours, she's also had a very generous allowance to give her a bit more independence as she told us was frustrated at feeling beholden to us for money when she was with us. She has burnt through this allowance every month ever since and when OH told her she wasn't going to get any allowance in December she told him she wasn't getting any presents for anyone, and she didn't. She didn't even so much as get a card for her Grannie who does so much for her its unreal. She topped this off by saying to her "I can't get you anything because Dad has been a knob". DSS by the way saved his allowance for christmas gifts and really enjoyed shopping for presents for all of us.
A month ago she had an Iphone 7 for her birthday, she asked everyone to combine xmas and birthday money and give it to her. She didn't thank anyone on the day when she opened it, not even by text. Our contribution was £250 to that. I wrapped a couple of very small stocking fillers and then stupidly felt bad and bought her some relatively expensive body lotion and spray. 5 minutes after she opened it and disappeared she tweeted saying how awful her day was and how stupid the conversations were downstairs. I'm really kicking myself for buying her something.
If she was my child I would have removed all the presents from her room, returned them to the shops for a refund and switched the WIFI off. OH went into her room and sat while she screamed in his face for an hour or so.
I guess my question is, what would you do in my shoes? I ended xmas day with an upset DSS, an OH nearly in tears and I didn't sleep on xmas night at all. In my view she is a total bully to OH and he takes it because he just wants to see his daughter. But nobody should be able to treat other people like that, teenager or not. DSS tells us she behaves exactly the same way to her mum at home, and on occasion her mum takes herself into her room to cry as she can't cope with her either. DSS told me on xmas day that DSD was being horrible about me a couple of weeks ago in front of his mum saying that I'm an unfit step-mother, DSS then confronted her and asked her why and she said "she just is". DSS has no problem with me at all I should add and I've never had a falling out with her.
Her mother also won't speak with OH on any level. So a team effort to turn this around is unlikely.
I'm of the view that all I can do is support OH but I really had to bite my tongue whilst seeing all my lovely family around me upset on xmas day. I can't get involved, I can't speak with her as it's not my place but at some point, I'm going to blow my top! Ultimately we've been round this loop for a few years now with ever increasing severity and upset for my OH so is there a way forward that will support him and stop her being such a bully to him?
PS - I came along years after the divorce and there was no third party involved then so he's not the bad guy in her eyes.