It's probably the opposite of brave to be honest, saying it on an anonymous forum! I'd never dream of saying any of this to DH as he'd probably, understandably, leave me!
And for the most part I'm happy enough to not damage my own children's childhood by leaving their dad simply because I don't love or have a bond with his older children. I'm not blind enough to say that isn't selfish, I know it is. But I have my children now and the best thing for them is for us to stay together. As I say, the thought of another woman feeling how I do about DSC makes me feel awful.
I don't even dislike DSC as individuals and we get on well. I just don't really have any strong feeling for them. I was naive at first at hoped that would develop in time but it hasn't and has only been heightened by having DC of my own which really showed the chasm of difference in my feeling or lack thereof between DC and DSC. And because I don't have strong emotional connections to them, I do prefer times when they aren't there. I find it easier to be myself, be relaxed and enjoy family time. Little things like feeling a pang of disappointment when it's our Christmas with them and the opposite, more excitement when it isn't etc.. It's not nice and I wish I didn't feel that way.
I think as you say, when two parents are involved where do you really fit? I went through all the feelings of being taken for granted by both DH and his ex when we first got together and I think due to that I did take a massive step back which probably didn't help the lack of bond I feel toward them but as you say, what do you do? You're not their mum but you should act like it whenever demanded? No thanks.
As I say, if it were possible to have my time again and still have my own DC (because I could never regret them), I'd never ever have gotten involved with a man with DC. But you can't go back in time and here we are so we plod on and I always try to stamp down these moments of feelings I have and put a smile on my face.
I don't think I'm a horrible person and I've never excluded them from anything, even when inside I have wanted to. I have always TRIED and I always will. But I can't make myself feel something that isn't there.
So yep I agree with the OP, most of the time blended families just don't work. Or at least don't work for one or more parties.