I find SD12 increasingly unpleasant

I would say build as happy a home life, despite her, as you can -for your sake and the poor siblings. We regularly read of adult DCs being similarly selfish. Fingers crossed she changes but she may not -you have 2happy DSCs enjoy them.

I have an SD who was like this, also got more one on one time that any other SDC, but it was never enough. Tried everything, love bombing, ignoring the difficult behaviour, asking her if anything was wrong, offering even more one on one time. Nothing worked. I ended up disengaging eventually, made plans when they were round etc.

FWIW she is the same as an adult, and the school flagged her as ADHD due to disruptive behaviour at around 16.

It’s not always just the blended family causing the issue.

Outrageous? You must live a very sheltered, highly controlled life!

Yes it is behaviour that needs to be corrected by a parent - but it’s not outrageous- she isn’t torturing small animals or telling wild lies to social services.

I think that's at least in part about disliking how you are with her

You come across as very unpleasant

She's punishing him because her parents aren't together. She wants him back with her mother. It's really that simple.

Everything you’ve said suggests she has attachment difficulties, likely attachment trauma from her parents splitting up and both her parents failing to suitably support her. What do her parents do to help her form positive attachments?

She's a very unhappy girl. Probably misses her dad when not with him. Is jealous because you are in his life the whole time, wants to punish you as she holds you responsible for her parents not being together ( whether that's correct or not is immaterial, she probably hopes it's going to happen). It's why she sits on dad's lap, makes a fuss about everything and annoys people. She feels that she does not matter. It might not be rational, but she's a child.
BTW I sat on my dad's lap, or snuggled next to him until he died. I was 32 when that happened. I loved my dad very much and had aways done that. I find that usual. If she does it when you are holding hands with her dad it's because she feels unwanted.
Edited to add, I also had a stepmother who realised that I was traumatised by my mother's sudden death and was mature enough to make allowances for me. We got on fine.

She sounds very insecure and unhappy. Does she ever get one-on-one time with her dad? You talk any you spending time/money on her but don’t say anything about her actual parent and what effort he makes.

She is desperately seeking attention - does she get much one-on-one time with her dad?

I do think that in these circumstances you have to keep trying OP. I realise it's hard but you can't treat her differently from her siblings, that's not right.

It’s not her, it’s you.

And you struggle to get on with a 4 yr old child as an adult? Probably should have left the relationship 8ish years ago. Poor kid.

Agree. She's only 12 for gods sake, still a child.

Are you serious??

What does your husband do when she is rude or interrupts?

I find your post rather sad. You met your partner when his youngest was just 1 years old and his middle child only 4. My youngest is 4. It's completely understandable that she feels insecure. As for not liking her all that much my own mum openly admits she didn't like me much either between age 12-15. I talked back and thought I knew it all. By 15/16 I'd come out of the other side of it. She persevered with me, always loving but had firm boundaries and I was absolutely called out on poor behaviour. It must be harder when the child isn't your own but you have known her since she was a little girl so I would hope some affection os there.

I think it is important to have boundaries re behaviour and manners as it is everyone's home and I would expect your partner to be on board and actively enforcing this.I do not think being polite but withdrawn is the answer.

Poor kid.

She sounds pretty unhappy. Can't your DH reassure her? She obviously feels she doesn't get attention from him/is unloved etc. She likely just needs reassurance from her Dad

It sounds like the problem here is not with your step daughter (although it’s completely understandable why you’d find these behaviours extremely trying), but with your DH. These sound like boundary and attachment issues, and ultimately it’s his responsibility as a parent and your partner to make sure that a) she’s getting enough reassurance, attention and one to one time, and b) that he puts boundaries in place that mean that his relationship with you and with his other children isn’t overrun by his daughter’s attachment issues.

If he keeps letting this slide then it’s inevitably going to build resentment towards his daughter from all the family members impacted by her behaviour, and this is extremely unfair to her.

Are you able to talk to him about the issues and ask him to both give his daughter stricter boundaries but also ensure that she feels safe and settled and that she has the time she needs with her dad?

How often does your DH see his DC? And do they all get one-on-one time with him?

It’s clear that your DSC12 is desperate for his attention, but also that this may be pushing his other two DC out of the way and in turn they may react in a similar way.

If, for example, your DSC only stay every other weekend, it may be that they just really want more time with their dad and by giving them (all of them) more bonding time, DSC12 will settle down a bit.

Obv if he sees them 50% of the time or more, you probably can’t increase the time he has with them (as I’m assuming mum wants at least 50%).

Nothing more creepier than a woman in competition with her SD 🤢