my observations on the step-parent/ex w situation.
I have been an exw for four years and my exh met someone shortly after we split and they have dts.
I don't think Im particularly bitter (she's welcome to him but things haven't been easy). Part of this I lay squarely on him doing anything for a quiet life and generally not stepping up to the mark with our dcs.
I also think she is blind to his character and he hasn't revealed his true nature to her yet. And of course no new partner is going to listen to an exp about his negative points. I wouldn't either. And is going to support him in his method of parenting and concerns.
Anyway he's never introduced us which I ve always thought was odd - in hindsight maybe I should have suggested it but thought it right he initiated it. he's an arch manipulator so it has suited him that we don't communicate.
For the first three years I took a complete backseat - liaising through him only - they took my dcs on holiday when dcs were 10 and 6 and each night locked the adjoining door which I wasn't happy about(given ds2 has nightmares).
He is always generally very down and critical on our dcs and constantly moans to me about them and used to ring me constantly when he had them to complain. I put a stop to that.
Although he sees them regularly it is never for more than 24 hours at a weekend which contravenes our agreement. When they played football he never went to matches - citing the dts routine as a reason. SO basically their needs have come second to new family routine.
Latterly dcs were coming home saying that his p was making snide remarks about me to them. From those it was clear that she had no knowledge of our agreement so ultimately I cracked and sent her an email outlining it. Also ds2 is a bit overweight and remarks were made by her to him about it [sad
backed up by exh.
The reaction to the email was of the 'how verydare you' /angry tone. I was told that an angry reply criticising me and our dcs was forthcoming however it didn't arrive. To bbe honest if my exh won't stand up for them then I'm going to.
I offered to meet up to discuss but no response.
DS2 says he is spoken to like a four year old and generally things there not relaxed.
They don't want to go - they're not just saying that to appease me - they mean it. DS1 has called ex h jekyll and hyde and that he speaks differently when they are own their own with them to when new p is around.
Of course all new p's are going to think the sun shines out of their new p's arse otherwise they wouldn't get together with them.
My impression of his p is that she tries to hard to create a 'family', it's overcompensating and the dcs see right through it. For the little time they see her and exh. I think it must be very hard and actually don' think I could do it. I'm quite happy to stay single for while and enjoy male company at a distance 
I must stress I have constantly pushed exh to see them more.
The most positive thing to come out of it is that they have a good bond with the new dcs.