Nothing's perfect around here either, but for the most part, it's working. DH used to be dependent on SS like that, but he's come around in the past few years and now understands that our relationship needs to be primary. I'm able to have minimal contact with SS's mom, so that helps everything run smoother. It helps that I get along great with my kids' dad and stepmom, so there's rarely a problem there.
It takes a long time to "blend," longer than I think most people can ever anticipate. Two adults who understand that the marriage MUST take center stage make everything else easier. Otherwise, the kids will always be tangled up in the marriage and that's not good for anyone.