I'm not meaning to be horrible or saying all step mums are the same, I would just like to get my head around something and hopefully one of you could help me understand this situation.
Myself and exdp separated 7 years ago, in that time hes had two relationships including one with the OW. The one he's now in a relationship is with the OW whom he cheated with on the OW he left me for Jeremy Kyle fodder right?!
He moved in with her last year and there was no space for our dc in her flat, so he ended up only taking them out for the day and brought them back home.
Now as the situation stands today, he hasn't seen or called our dc in over a month, despite me calling/texting asking what's going on, the last time i called him was two weeks ago when she heard a female voice, she started screaming and shouting so he hung up immediately and I haven't heard from him since.
This morning I find out from a mutual friend that hes cheated on her surprise surprise and she is refusing to let him out of her sight, which in turn means he's not allowed to be around me to see our dc
because she doesn't trust him. Now I know this is all down to him and he should have more of a backbone, especially when it comes to our dc as it is affecting my dd8 who thinks her dad doesn't care about her which isn't far from the truth 
But how do I handle this? The lady that he's with has given him an ultimatum it's either her or our dc as she wants to move away and start her own family, so it's pretty obvious he has chosen her and hasn't got the balls to tell me that himself, but how do women do that? Know that he has children and is prepared to sit back and watch him abandon them in favour of you to then bring more children into the world? Its shocking.
She has never fully accepted our dc and everytime he was with them she would be calling to see how long he would be, he even walked away from ds sports day because she kept calling.
My own dp who I've been with for 3 years has fully accepted my dc and does more for them than their df, if he ever made me choose there would be no question over who i would pick and it would always be my dc.
I honestly don't understand the mindset of some people.