I recently had the opportunity to plan his surprise 65th birthday party. We made picture boards and wrote wonderful memories to go with them. The look on his face was unforgettable. You can't replace their missing parent, and shouldn't try. Just be there for them as much as you can and don't expect miracles.
Love takes time.
You can love many different people in your life. You might keep in mind that your stepparent chose to be a part of your life, they didn't have to. So give them a break and try being nice to them.