To extend means both to enlarge and to reach out. An extended family is both larger and more encompassing of other people.
The shape of the American family is changing for the better, becoming more inclusive, more diverse, and more extended. Shared custody (both legal and physical) is becoming more the norm, and unlike in the past—when many biofathers left the scene—biodads are often very involved in parenting their kids after a divorce. More involvement means more adults in parenting roles and far more well-combined families.
The only problem with this improved state of affairs is that the more people there are involved in any activity, the more time it takes to plan things and to negotiate through differences of opinion. (Hey, as far as I'm concerned, this caffeine-based, gotta-hurry generation could all use a little more slowing down and time-taking anyway.)
The Value of the Extended Family
Children need other people (don't we all?). Kids do best (and parents, stepparents, and families survive intact) when there's a support network of many people, including relatives, adult friends, teachers, and members of the community.
In any community, the people who live there are mostly luck-of-the-draw; you don't get to pick the residents. You don't get to choose your partner's ex, either, or the family of your partner's ex (nor did they get to pick you!). Nonetheless, all of these people are a distinct part of your community now. Recognize that they are part of your stepchild's (and therefore your) support network, and you are part of theirs. The more you're able to see the wider picture and accept the abundance and diversity of this network, the easier it will be for you to rise above individual disagreements.
All generosity—both monetary and emotional—in a stepfamily is ultimately in your own best interest. Some people call this Karma, some call it the Golden Rule, and I call it True. You get as much as you give!
You can say it's for the sake of the kids, but the sake of the kids is your sake, too. It's nearly impossible for your stepchild to bond with you when your obvious dislike of her other bioparent gets in the way. She'll feel that getting close to you will hurt her biomom or biodad.
No! You don't have to get all buddy-buddy with the ex now. A working, respectful relationship doesn't mean beer dates, bowling, or heart-to-hearts. Your stepdaughter's soccer coach is also involved in your community, but you don't feel compelled to borrow clothes or tools from her, do you? Work on developing a practical partnership with the ex, not a close friendship.