Dealing with a Motherless Daughter?

Will try to make this short. My stepdaughter of 4 years hasn't had much contact with her mother at all. Months and months will pass without contact. Well, now, Mommy has decided to make a grand re-entry, and my lovable stepdaughter is wreaking havoc. Mommy says that she doesn't have to tell her dad (my DH) what goes on when SD is wiht Mommy. This wouldn't be a problem, except for Mommy has a bad habit of lying and blaming her problems on us. SD comes home from a brief visit and hates us. She talks to Mommy on the phone and suddenly she no longer has to tell us what's bothering her or why she's so angry. All respect has gone out of the window. I hate that her mother puts her into these situations and manipulates her so easily, but our little one is really starting to worry us now (she's only ten). We find her going through our cell phones, etc. Wouldn't be a big deal, except we don't want SD to see nasty texts her Mom has been known to send (keep the kids, I don't want them, etc.) Seems like the more we try to help SD open up, the angrier she gets and the more she lies to us to cover for her Mom. Sickening... help, please? Nearly at Wit's End. ANY advice more than welcome. And the "she will grow up and it will all make sense one day and she'll appreciate you" thing is no comfort right now. We need advice ASAP on best way to handle her- we understand the hurt and pain, just trying to find a way to balance that with maintaining respect and rules...