Get your 
and 
ready, long post to avoid drip feeding. Long time lurker, first time I'm posting.
Amicably split up with ExDP when DS was 4 and DD 6 months, was pleased that he moved on pretty quickly, but had issues with his new DP and the way she acted around my DCs straight away. Over the past 2 year she has become un-fucking-bearable and I'm at my wits end and clueless as to what to do about her. She was told at a young age she couldn't have children as she delights in telling everyone she knows repeatedly at every chance she gets and it's obviously been a major issue for her all her life, she started IVF privately less than three months after getting with my ExDP, and is constantly trying to undermine me and take over the care of my DC's.
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Two weeks after he began to date ExDP she had pictures of my DC on her facebook, one of which was of her bottle feeding my daughter (I didn't even know they'd met her at this point) with the tag, 'mummy time' others has tags like 'my handsome boy' I immediately texted ExDP and told him to get her to take them down. She set her facebook to private so I couldn't see if she had them up or not and then sent me text messages telling me she was just trying to bond with the children and that I was being unreasonable.
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I invited her and ExDP to DD's first birthday party, she spent the entire day monopolizing DD refusing to allow anyone else to hold her, insisted she was in every photograph with her and even tried to open her presents with her. When I put my foot down and refused, she told me it was unfair that I was excluding her and stormed out of the house crying.
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Convinced herself and ExDP that DS had a wheat allergy, no idea where this came from as he has never had any issues with allergies, started him on a wheat free diet at her house and tried to make me follow it, phoned up his school behind my back to tell them he had this allergy and she was worried that I wasn't taking it seriously enough.
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She took my DD for her first haircut w/o asking me and is constantly taking DS despite me repeatedly asking her not to cut his hair. She has kept DS off of school to take him to unnecessary Doctors/Dentist appointments w/o asking me, I only found out when I received absence letters from the school.
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Bitched, whined and moaned from day one that ExDP doesn't get enough time with the DC, despite the fact he suggested our current arrangements and has never said he's unhappy with them.
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Sent me nasty messages when I went back to work and chose to put DD in a nursery instead of allowing her to look after her during the day.
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When DS starred in the Christmas Nativity she demanded I force my DM to give up her ticket to her so she could come see him (there was a limited number of tickets per child), when I refused she accused me of trying to 'keep the children out of her life'. My DM had terminal cancer; she won't be able to see him perform again.
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Constantly feeds the DCs chocolate and sweets, knowing I'm trying to limit them, encourages DS to lie to me about this.
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Tried to start potty training DD when I specifically asked her not to, as she wasn't ready for it.
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I'm still very close to ExDP's PIL they think she is a weirdo they invited me to a family BBQ, I left children with a close family friend for the night. She spent the entire night making comments about how she would never leave them alone and said I was selfish to put a night out before my DC.
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ExDP couldn't make it to Parent's Evening so she came instead; she repeatedly interrupted the teacher and ranted about how the school was not doing enough to support DS's learning. I have never had an issues with his schooling, I was absolutely mortified and had to apologize to DS's teacher.
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Threw a massive tantrum in front of the DC during handover and made them cry when I told her explicitly she would not be taking them out of school/nursery to go on honeymoon with her and ExDP for 2 weeks to Australia DS still hasn't forgiven me.
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MIL phoned me and told me she'd invited ExDP and his DP over for a meal, apparently she'd spent the night with ExDP discussing the likelihood of them getting custody of the DCs if they went to court!
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Is CONSTANTLY buying them expensive presents turning them into spoilt brats which they can only play with at her house. DS now tells me he wants to stay at his Dad's because it's much more fun than here.
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Found out I hadn't breastfed my DC and spent a family party telling all who would listen, had they been her DCs she would have done it because she's not selfish and cares about their health.
This all came to a head tonight at handover when she revealed she's had the DC's names tattooed horribly on her chest. I admittedly lost my temper and told her she needed to stop trying to replace me and that my kids are my kids, to which she replied she was a far better mother than me and it was unfair that she couldn't have kids and I could.
I honestly cannot deal with this headcase woman anymore. I've tried to raise this issue with ExDP and told him that I feel like she's trying to replace the children she can't have with our DC, but he completely supports her and thinks I am being unreasonable and that it's unfair of me to not let her have an equal say in how they are raised. I've warned him that if she doesn't back off I'm going to stop contact, I have no idea what she's saying to them when they are at his house. As I said, I'm not the only person who sees this, the majority of his family agree with me.
AIBU?