Okay so first, an 11 year old acting out sexually would imply that she may have been sexually abused in the past, or been exposed to things she shouldn't have been. Don't put adult motives on her - she's 11. She doesn't know how to "sexually manipulate".
What you do is TELL HER MOTHER. Don't put any blame on the child. Share your concern, which you may have to dredge up from somewhere, since you express none, but she deserves. You tell the mother that you think her daughter needs some counseling because she is expressing sexual things she shouldn't know yet, and under no circumstances, for her sake and yours, are you to be left alone with her.
You lock the bathroom door.
She IS a sweet and normal child. She may have been abused, and this is no fault of her own. I'm sort of stunned that you think a child is capable of such manipulative behavior, and that your thoughts didn't once go to "Where did she learn this at such a young age?"
11 year old like that ins't 100 thousand per cent innocent. When i was 11 i knew the difference between lying and getting someone in deep trouble and being honest about something. Stay away from her lock the bathroom door and collect evidence in your favor when she threatens you that way like audio recording her threats. Tell the mother to have a talk with her.
Suggestion of auntijessy appears to be okay and practical. He must be on very good terms with his girlfriend and he has to convince her that her daughter needs professional advice .It is obvious that the girl had some sexial experience with her mom's boyfriend.Professinonal advice is a must.