As strange as it might sound, it is possible for a step-child to feel both joy and sadness when a parent remarries. The remarriage signifies both a gain and a loss, for the child.
Older children are generally less accepting of new parental figures than are younger children.
Older children may have had a longer period of time to attach to a biological parent and therefore feel a greater sense of loss. Even when biological parents share custody, children experience and re-experience feelings of grief as they move back and forth from mom's house to dad's house and vice versa. Parents in a step family also experience grief. There may be unresolved issues of anger or hurt towards the ex-partner that carry over into the new relationship. Feelings of guilt, of traumatizing one's children or hurting an ex-partner may stir up more grief.