We have had issues with DS, but for different reasons. DS's therapist recommended we get a keyed lock for the bedroom door, which we have done and use ALL the time.
Because it is only happening to you and not your DH, I can understand why he may not take it as seriously, especially since she is his child. As for why she's doing it, it could be for a myriad of reasons. Are we talking only about 'feminine' things, or is it possible for her to take something comparable of your DH's and takes yours only? If that's the case, then she seems to have an issue with you. Do you have any idea whether she does this with her BM as well?
In the meantime, keep using the lock and make sure that your DH uses it as well....at ALL times, even if you are only going out for a minute and she's there.