Probably both NADM - I'm realistic, I don't expect DP to love my kids as his own, but I guess when you see inequality in the way they are treated it is bound to bring out the primal protection urges in you. I tend to over-compensate and almost end up favouring his kids so as not to show bias, which can end up upsetting my own DD, its a bloody minefield.
When the kids are putting on little shows & performances I can see him looking at me all proudly, as if to say "look, isn't she just perfect?!" Erm, actually I'm busy concentrating on how cute my own DD is!!
He actually asked me to smell her once because "she smells so amazing" - FFS I am not sniffing someone else's kid (don't particularly want to sniff my own) - as far as I'm concerned, kids generally smell of dribble and whatever they had for lunch!
I want to point out that the only other person who will look at his DD in the same way he does and love the smell of her cheesy feet is his ex and if he wants to share adoring glances with someone, it will have to be her, but obviously I don't really want to go down that road, so I just try to ignore it and when the moment arises I politely point out that my DD is my little angel too.
Not sure that it is a deal-breaker for the relationship, although it has caused several arguments and it has made me realise that like you financialnightmare I couldn't live with them. Shame, as he is perfect in almost every way and I could even put up with the fact that he's a messy bugger as he makes me feel so happy and loved, but coping with someone else's kids/parenting 3 or 4 days a week would be too much.
Sorry letting off steam here, not helping much am I OP?!