Clearly, I'm new at this...

I realize I'm making rookie mistakes but I haven't the faintest idea on how to proceed. I recently became the SM of twin toddlers. Yes, twins... a boy and a girl. I have no children of my own. My own parents divorced and remarried -- as a result, I am the oldest of six children within a blended family.

Since I was raised in a blended family, I had some insights on what I could expect. Before even combining our households, DH and I spoke at great lengths about his divorce, the changing relationship with the BM, how that was impacting his children, where I would fit into this, etc. I tried to start this with an eye on a strong foundation, but not surprisingly, some aspects of starting a family are never predictable.

We are all learning how to live with each other. There have been plenty of moments of exasperation but we address each of them as they arise and none have gotten out of hand. The SCs and I get along great and when they do test their limits (as I would expect toddlers to do!), we act together to curb bad behavior. To respect the co-parenting arrangements with their BM, we have agreed that any discipline beyond time-out measures will be handled by him. All parties (including the BM) seem to be happy with these arrangements.

I enjoy the time I get to spend with them but peripheral issues are starting to get troublesome. I have tried to be careful about boundaries -- parenting limits, household chores, contact with BM, expenses, transportation, etc. But it seems like as soon as I vocalize where the boundaries for me are, they are promptly tested.

To illustrate, I present you this scenario: DH is called into work early and he is scheduled to have the kids later that evening. DH relays this information to BM and she reacts poorly, tells him that she is dropping the kids off even earlier than the previously arranged time. (As a side note, BM is an ex for a reason. I believe in respecting her role as their mother, but she's, well, uh, difficult to keep it polite...) DH says to BM that this is fine and ends the call. He then tells me that she will be dropping the kids off after he goes to work.

I am not pleased. Though I have been willing to assist in these tight spots before, he's always given me the courtesy of asking to watch them -- this is the first time he'd taken my assistance for granted. Previous arguments with BM and he resulted in a boundary with me that I would stay *very* distant from such exchanges. If they were going to disagree on parenting issues -- it's not my place to get involved.

That aside (I told him that it would not be in either of our interests for him to 'volunteer' my services like that again and he's agreed), BM brought the kids over during what should have been their nap-time. They were screaming, crying, and in quite a state of agitation. BM and her fiance arrived with each of the twins in their arms. She says to me, "I can put them in their beds for you." I tell her, "No, that's okay. I'll take care of it." She ignores me, pushes past me and into our house, crosses to the opposite side of the house to their room, and takes over the first floor. I waited for her to get the kids to their beds (still screaming and crying), and let her know that I could take care of it from there. She continued to ignore me and stayed another half-hour while her fiance hung out in the living room. They never did stop screaming until she left. I was able to get them to calm down and nap within 10 minutes of their departure.

This isn't the first time she's helped herself into our home. She's established that she doesn't want either of us in her home and we've respected that. BM doesn't seem to think that those arrangements should be reciprocated because they are *her* children. (Or they are "their" or "his" children. It depends on which angle is most convenient for her at that moment.)

Boundaries. My involuntary babysitting is something that I was able to talk out with my partner but the boundaries with her are more difficult. I have requested that he step up to the plate and help shape these boundaries but she is always pushing, pushing, pushing, and threatening.

So please, is there any advice you can give a newbie for dealing with this situation?