Okay, I think in this instance I am possibly BU. Didn't want to drip feed but am typing on my phone so bear with me.
DP and I don't live together so I feel a bit weird calling her DSD. But I love her to bits and think it's great when DP has extra time with her. And I know he loves it too. That's not the issue.
His ex has a lot of form for asking him to have their DD on weekends and extra days at very short notice. From June to September she had her DD a total of three weekends. Not for any good reason other than she wanted to go on holiday, it was her birthday weekend, etc.
Somehow, whenever she's ill/wants to go away for a weekend/cant find a babysitter/whatever then she always expects my DP to change his arrangements to look after DSD. But whenever my DP has something come up and needs someone to look after DSD (which is very rarely because he's reliable) then his ex tells him that his days are his days and he's got to sort out DSD himself.
It's the double standard that pisses me off. I know for a fact that there's no way she'd have DSD for an extra three days if DP was ill.
To answer a few questions some have asked: he is the primary carer when you add it all up on paper. He did ask for it to be formalised but his ex wouldn't have it. She kicked up a big stink about wanting it to be 50/50 and then promptly booked a weekend away on the weekend she was supposed to have DSD.
DP never asks me to look after DSD. He's a brilliant father and partner.
There was one instance a few weeks ago when his car broke down a few miles away from home just before he had to do the school run. So he asked me to leave work early and go pick up DSD while he waited to be towed. That's the first and only time he's asked me to do anything like that. But he didn't even bother calling his ex first (she lives right by the school) because she wouldn't have helped him out.
The thing is, he can't tell her to piss off because every time she does this it's extra time for him with DSD. So she keeps getting away with it. But I just think it sends a message to DSD that her mum can't really be arsed. And it takes the piss out of DP's kind nature.
Sorry. Bit of a rant there!