Not sure Dad is the one to handle this. It's not his stuff the kid is stealing and he'd be too chicken sh*t to report her.

Put a camera on kid's bedroom door so you can see when she leaves her room. Put some in other various areas so you can see where she went and what she is doing. Report the little thief as soon as you get her on video doing what evil little deed she's been up to. Let her know she will be reported for the little thief she is, and then follow through with it. Sit Dad and her both down for this chat. Make it clear to both you will report and press charges. 

Try to keep thing locked up. When a kid is this way it’s very little you can do.  Keep her out of your house. ?

Nanny cams everywhere. If she brings in a purse or backpack, they must be checked before she leaves. Jacket and pants pockets must be turned inside out to prove she has nothing but HER things or lint.

This is what my DH did after PigPen stole from us.

I admit I am an a$$hole I would get nanny cams and place them all over the house, I would also call the cops once I had the video evidence of SD stealing the stuff and have them come "counsel" her.

I'll share with you what we did to SD when she posted and shared naked photos online:

Since SD was a manipulator and a liar we knew she would never confess the truth even thought we had the evidence online.  Instead we had our cop friend sit down with her and tell her that her photo was found on a pedophile website that is monitored by the police and when he saw her he reached out to us immediatly.  He told her that he was not going to press charges on her for wilingly sharing her photos to a pedophile porn site, but that she was now on a watch list.  Aside from SD confessing everything and being scared sh!tless, somebody was able to get through to her that doing that kind of crap was no joke!  

Just some food for thought.

A child psychologist told us IF a kid steals or runs away call the police.

Its an awful feeling to be violated inside your own home. WE know what it feels like.

Lock on your bedroom door, install cameras, and the next time something of yours goes missing, something important to SD needs to disappear. *diablo*

Don't hestitate to email BM. Be direct and tell her that SD has been stealing. Include a list of missing items, and tell her you'd greatly appreciate her ensuring their safe return.

I'm not suggesting this will get your stuff back, but it will drag the issue out in the open and mummy and daddee wont be able to cover it up like something in the cat's litter box. BM will explode, then contact your H, who should already be hurting from having your bi!ch boot buried up his butt. Let him know that his life won't be worth living if he doesn't talk to his ex and work with her to straighten out their little piece of work.

Being nice rarely works in steplife. If you don't make your H more afraid of you than of BM and their light-fingered Lolita, you might as well grab a sharpie, write WELCOME on your forehead, and lay down by the front door. Divorced parents will often ignore bad behavior, or don't understand why we have a problem with it. Dump the stealing in the parents' laps, shine a bright light on it, and make sure that your H handles his baggage. Don't allow this to be ignored or rugswept.

And FWIW, my DH and I went through H@ll with his youngest, but to this day we say "Well, at least she didn't steal." Thieves are the lowest, even in a dysfunction-rich step dynamic.