Help! 11 Yr old SD problems

[RRach3]

Hi I'm new to this so hope this message in the right place.
Can anyone in a similar situation with an 11/12 year old daughter offer advice please?
My partner and I think biological mum is bad mouthing "stepmums".
We are now getting married (June 2019) and SD is acting up. Calling her dad by his name not dad. Physically placing herself between us at every opportunity, on a walk, stood in the kitchen etc. Ignores any conversation about wedding. I've had the "your not my mum" quite randomly with no context. My mum this and that. I so far have said very little and made no upset or angry comments but am becoming frustrated. We are trying to be extremely patient with her and understand that she sees the wedding as "a loss" to her life psychologically. I think if her mum could say "it's OK to like SM" SD wouldn't feel disloyal. I just want her to be happy and us all to get on when she visits. Not to be another mum.
Any advice would be much appreciated.

RRach3 I'm so pleased things are working out well for you, well done :)

Sounds like his ex still has some serious issues, but that's her problem, not yours or your DP's. It's incredibly hard for your SD at age 11, but with you guys presenting a loving, supportive and united front it will be ok. Eventually she will see the problem for what/who it is, and that will be hard for her too, as it is for everyone when we realise our parents are people with faults and flaws just like everyone else, and that accepting that is part of growing up. When she gets to that point (and it will happen), she will realise that the only person who can help her Mother with her issues, is her Mother.

We are just getting to that point with my eldest SD. In a nutshell, we had a rocky start, then things were really good between the 2 of us, then she had to join the adult world, which she didn't want to do, and I was back to being the bad guy and she went to live with her Mother. That was 3 years ago. She is just now beginning to realise that actually, I'm not the bad guy and actually, her Mum needs to own her issues and that no-one can do it for her. SD spent the weekend with us. Frankly I was dreading it. Yes it was still awkward and draining, but it was better than it has been. Small steps at a time, we're here for the long haul :)