I myself am not a parent, but I can tell you that me and my stepdad did not get along for quite sometime. We are very close now that I'm older but as a young child having someone new in your life can be very difficult. To form a stronger bond so you can have the ability to relate well to your stepchildren I suggest that someone should make sure that you supply adequate, but not forced bonding time. If you make them do something they will look at time with you as an obligation instead of what you're trying to attain. Never try to be a replacement to their original parent as this is an impossible goal. They will always think about their original parent so do not try to silence them when they need to talk about them. Encourage it and welcome it. Talking about it may bring you closer together and be glad that they're willing to open up and use this as a time to become more involved.
I don't know if this is exactly what you're asking, but I hope it helps.