The fact is that the biological parent’s gain -- a new romantic partner – is often experienced by his or her children as yet another in a series of family calamities. Remember that the stepchildren involved in remarriage situations have undergone early losses and often stressful transitions, such as a move out of the family home and to a strange school.
Also, the initial loss of the intact family – most likely due to divorce – has been experienced as a volcanic upheaval, inevitably bringing deep grief and fears of abandonment in its wake.
Moreover the stepchildren are often struggling with loyalty issues: the guilty feeling that harboring liking – or even outright loving – feelings for the “replacement parent” is a betrayal of the “real,” biological parent (in reality, or in memory if that parent is dead).
What to do? It is helpful to limit the number of new rules and changes at the outset of the new marriage. What changes you do make should be minimal and focused on maintaining civility. For example, a stepchild should be required to look at the stepparent and say Hello when entering a room rather than greeting the biological parent and pretending that this new person (the spouse) is not present.