Both research scholars and clinical experts agree that the stepparent role is more difficult and less clearly defined than the parent role.

However, I have no legal rights, obligations, or duties, financial or otherwise, towards DH's children. Any rights, obligations, and duties I take on are derived solely from DH in sharing his rights, duties, and obligations, and are agreed upon solely between DH and I. Our collective values of home and family are in keeping with entitling the children to be properly cared for and nurtured in an environment free from neglect, endangerment, or abuse.

With that said:
~I am available/able to assist DH with their children during HIS parenting time.

~I have limited availability/ability to assist TM and their children during HER parenting time:
1. for an acute medical emergency.
2. during my non-work hours for transportation issues arising from vehicular failure or breakdown.
3. to provide COMPLETE respite care for a mutually agreed-upon period of 24-hours or more during exercised ROFR.

This is from:
http://www.steptogether.org/disengaging.html

Your SKs are not your children.
You are not responsible for overcoming their previous "raising."
You are not responsible for what kind of people they are.
You are not responsible for what kind of people they become.
You are not obligated to become an abused member of the household just because you married their dad.
You are not responsible for raising your SKs.
All the responsibility belongs to your DH.
Your DH is not a mother.
Your DH is not going to raise his children the way you want him to.
Your SKs are not going to turn out the way they would if DH supported you.
What all this means is this: You must stop parenting your SKs. You must stop telling them what is expected of them. You must stop disciplining them. You must turn over all responsibility for them to your DH. You must allow DH to make whatever mistakes he makes.