If you're a quiet, shy polite person who's maybe never come up against manipulative controlling bonkers behaviour before, it can be incredibly stressful and upsetting.
I'm so sorry you're having to deal with this as well as new baby, hormones and healing and the different world when two become three.
Having to create a strategy at a time which could be calm, coccooning, supportive and kind isn't fair.
And your DH isn't being fair either.
Your family unit is mother&baby + father. It doesn't include an entitled intruder which is what she is.
He's grown up with this so unfortunately it's embedded in his mind to enable her. They're both used to it, this is their normal. Do you want it to be yours and your child/ren's?
He should have seen it and dealt with it but hasn't, so you do have a DH problem, you shouldn't have to be dealing with that right now, yet here we are.
Time for him to step up, take his number one role of husband and father with you, as the centre of your story.
I'd recommend writing everything down.
Remind him that you and baby are his focus and centre now.
How you're feeling after the birth.
What you and the baby want and need from him now - to make this stop:
The snatching and witholding when you need to be close to the baby.
The attempts to undermine you as mother when you need to be recognised and respected.
Scornful behaviour towards your family, when you need harmony and support.
Set it all out calmly and clearly, how her behaviour makes you feel, what needs to stop and what you require from him - to speak with her and tell her what needs to happen from now on.
Remember - you have RIGHTS. And your status has changed - being a mother gives you duties and responsibilities.
So you need to step up too - it's going to be hard and will involve changing how you think, feel and behave.
Breathe from your diaphragm not your chest - it will give your voice power and resonance, make you feel calmer, stronger and supported.
Practice saying NO in the mirror. Calmly and assertively. Eye contact and a neutral expression and tone of voice.
Practice stating what is going to happen. Give baby back now, it's time for me to feed, change etc.
Practice the three part sentence for assertiveness:
I understand …
However …
Therefore …
I understand you enjoy holding the baby.
However it's time for me to feed her now
Therefore I'm taking her to the bedroom.
NO.
Practice the broken record assertiveness technique. Repeat yourself, same words, same tone of voice, same expression, no more information. Nothing she can seize on.
Just on repeat - and make sure others can hear and observe what you're dealing with.
Then I'd also recommend reaching out to your own family and friends, if you haven't already, setting the same issues out and asking for their support, and what you need them to do if she starts acting up in their presence. Agree a strategy with your support network.
Asking for help is the first step to getting it. And doing it in writing can be easier if you're not used to it.
All the very best. Let us know. XX