Hello K.,
I would suggest you sit down with your husband and discuss what your role should be to the stepchildren. If he agrees that you should be part of a disciplinary unit, then the next step is to sit down with your husband and write down rules you expect all the children to follow while in your house. If you have a good relationship with the birth mother, you could try to synchronize your rules with hers. This is often very difficult, though, as you'll find it's hard for any two people to match up on parenting styles, let alone three.
Once you have defined rules, you and your husband should sit down with the children together, and you should both make it clear to the children that these are rules that you came up with together, and that you both expect to be followed. It might be good to have your husband do a bit more of the talking if his children are giving you more trouble.
Your stepchildren are bound to have discrepancies in your house rules and their mother's house rules. Be consistent, but understand that it is difficult for young children to adjust back and forth on house rules. To help with this, when the children break a rule after coming back from their birth mother's residence, give them a warning and ask them to repeat your rule back to you.
It's hard for stepmothers to live under a shadow of the birth mom. Understand that you need to parent the best you can, and if your stepchildren feel the need to complain to their mother, there is nothing you can do to control that. Try to work with your husband and share your concerns with him.
M.