The real difficulty is in coming to the understanding that the children in your new family now have two kinds of parents. The children involved each have one biological parent and each has a new step-parent. As adults your relationship took time to develop. You met, dated, fell in love, planned a wedding and married. Even if the children involved were a big part of all that time, once you married the clock may have started over.
Children take time to develop relationships.
Don't rush the process. If a step-parent is patient and loving, most children will also slowly develop feelings of attachment and caring. In the blended family this is especially true. Both parents must find a way to trust each other. Through communication and understanding, a trust for the individual style of parenting develops.