If there are kids, partners to a remarriage do not get a developmental
period as couple before they are parents. And then, because it takes time
for family feelings to develop, that bond is immediately under assault by
the children. For that reason especially, every family expert recommends
that couples heading into remarriage prolong the period of courtship
despite the desire and the financial incentives to merge households.
.............
People need to develop "a deep empathic understanding of the different
emotional worlds parent and stepparent occupy." To be a stepparent,
Doherty adds, "is to never be fully at home in your own house in relation
to the children, while the original parent feels protective and defensive
of the children. Neither 'gets' it until each describes what the
emotional world is for him or her." Each partner is always an outsider to
the experience of the other
From
http://www.smartmarriages.com/remarrying.html