Found this and wanted to share:

• "The kids are hostile and rejecting no matter what I do. I know it's not their fault. But it's as if I'm not supposed to have any feelings about it, let alone discuss them."
• "I can't do anything right--if I buy them a present, they think I'm buying their love and if I don't, I'm cold and unloving."
• "My husband doesn't have many rules--so I look super strict and mean if I ask them not to eat with their hands!"
• "Their mother says unkind things about me and calls every half hour while they're here. So it's hard to build a relationship with them."

These women were no whiners--most had been trying to get stepmothering right for years, and all began their stepmothering journeys committed to forging a great relationship with his kids, whatever it took.

But they're correct that there are external forces, most beyond a stepmother's control, that may undermine her good intentions and best efforts with his children. These factors include loyalty binds, a child's jealousy and resentment, the Ex Factor, permissiveparenting, cultural expectations about women and children, and a phenomenon called conflict by proxy.
In spite of such obstacles, there is a widely-held notion that "If she's kind, they'll warm right up to her." "Just remember," one "expert" advised in an online article, "You'll get back what you give. Keep loving them."

In this formula, the only good or successful stepmother is one who is embraced by her stepkids. Here's why that standard is so off the mark--and why kids of all ages really dislike their stepmothers.

Loyalty binds. Many stepkids--and adult stepkids--suspect that liking stepmom would be a betrayal of mom. So they keep her at arm's length--or worse. And there's nothing she can do about that. Only mom can release them from the torturous loyalty bind and pave the way to a healthy stepmom/stepchild relationship, by saying, "I wish you'd give Jenny a chance. I won't be upset." Too often, no such permission is given.

When there is a loyalty bind, nothing's worse than stepmom bending over backwards to win the kids over. Drs. Larry Ganong and Marilyn Coleman found that such stepchildren and adult stepchildren are especially rejecting of a stepmother they find warm and appealing, as she elicits tremendously conflicted feelings.
Possessiveness and jealousy. Children may become remarkably close to their parents post-divorce, and used to having mom and dad "all to myself." Adult children may develop an intense, peer-like relationship with a single parent, making adjustment to a stepparent tough. With a preadolescent or adolescent girl, possessiveness and jealousy will pose an even bigger problem, psychologist Mavis Hetherington found. In her Virginia Longitudinal Study of families who divorced and remarried, preteen and teen girls especially described the stepparent as an interloper in their world and an obstacle to intimacy with mom or dad. A stepmother may encounter particularly fierce resistance from a teen girl, both because she is close to her father, and because teen girls tend to model the feelings and attitudes of their mothers.

The Ex Factor. While there are exceptions, an ex-wife generally poses more challenges for the stepmom/stepchild relationship than an ex-husband, stepfamily experts Constance Ahrons, Anne C. Bernstein, and Mavis Hetherington found. Why? Mom's more likely to be the primary parent, and to have a strong agenda about what goes on in her ex's household. The stronger the ex's agenda, researchers found, the more involvement across households--and opportunities for conflict. And high conflict situations between two linked households lead to greater resentment of the stepparent, who feels more expendable and less loved by the child than a parent. In addition, Hetherington found that ex-wives feel more anger, and feel it for longer, than ex-husbands. Stepkids pick up on these feelings--and often act them out on mom's behalf. Translation: stepmom loses this draw due to gender.

Permissive parenting. Research consistently shows that children do best with authoritative parenting--high levels of warmth and high levels of control. But post-divorce, permissive parenting (high warmth, low control) frequently prevails. Why? Mom is likely to have primary custody, and if she's single, that can mean a lot of work and stress. She might let the little things--and then the not so little things--go. Dad likely fears that if he angers his ex or the kids, he won't see them as much, and feels guilty that the kids went through a divorce. And so an "Always ‘Yes' Dad" is born. Against the backdrop of permissive parenting, stepmom's normal expectations about manners, scheduling and respect may seem draconian, rigid, and "unfair." And kids with permissive parents understandably don't have much sense that it's wrong to be rude to an expendable-seeming and "overreaching" (in their view) stepparent. This ticks off stepmom, who then seems even less likable and fun to her stepchild.