hello I am bookmarking this thread.... it's Great !....
I'll call him DH for the sake of continuity, but we're not married as he's still going through Divorce proceedings with his now Ex.
I get on well with the DC - no problem, they live with their mum and come to us every other weekend. I am adverse to calling them step children ........
We have been flexible since day 1 - whenever the Ex needs to revise weekends or whatever cause she's going out - no problem.
We have arranged a couple of weekends - which may need some flexibility on her side (plenty of notice) - and we're accussed of changing the weekends 'all the time'.
Flexibility is key surely in these situations if it allows.
I tried with the Ex in the beginning - but after 3 years, and her blatantly blanking me one day not 2 yds away from me when I said GM to her - she can fuck right off, plus when we're together and she's speaking, and maybe I say something - she will just reply back to DH and pretty much not acknowledge me 
so I don't try anymore, and just ignore her. I can't even bear the thought of her in my house....
I get annoyed that my DH won't stand up to her a bit more, but sometimes mums (and that's not all mums!!), use the kids as bait to get what they want, when they want - and it infuriates me and DH.
I concur with loads of others about being seen as second rate, which I think it's increased further when you haven't actually given birth - and my opinion not counting etc etc......... it drives me insane...
I want to invest the emotional side as much as a 'proper' parent - but it can be taken away in a nano second..........
I'm wondering when the Divorce goes through if it will get any better?? Probably Not.....Or maybe if the Ex finds someone else....
I feel sorry for the DC's because it's clear that they won't interact with me in the same way when their mum is around - it must be horrible for them having to tip toe around so as not to upset their mum. 
Ho Hum..
Rant over - and thanks for reading