I can't imagine how I'd bring it up with DS that his step mum doesn't love him as one of her own... that would be awkward. She doesn't have her own though thankfully.
Unfortunately for my DS he doesn't have two biological parents who would move heaven and earth for him - he has a mother and 'step' father (we're not married) who would do that, and a dad who couldn't give a poo. I do think that in situations where children have two very involved parents a step parent could take a supporting role, as opposed to taking the child on as his/her own. I really feel if I had support from ex with regard to discipline, schooling, life choices, etc I wouldn't need to rely on DP so much for it.