J.F.
It sounds like your stepdaughter is behaving in a way that is not surprising, but does need to be addressed. It seems like there is a lot going on around her, you are a very active, involved mother with commitments that while directed towards your kids, still distracts your attention from being entirely on them every second. You also have two children that need a lot of attention directed toward their specific needs. She may be acting out, especially at family functions because there is a part of her that feels she is lacking attention, plus she probably thinks that she looks older and more mature to the people around her. Or, if she's acting up in front of YOUR family only, it could just be because she wants to be rude to YOUR family, which is also unacceptable. When "interacting" with her dad, she probably thinks that she's behaving in an adult way for them to bond, and also to distinguish that her relationship with him is different from his relationship with her siblings. My half-sister is 45 and still to this day is border-line freakish about time and interaction with our dad....but then, no one ever called her out because they were afraid of hurting her feelings. She has grown into a very insecure woman with a lot of issues because she was never taught how to appropriately feel or react to certain things in life.
Perhaps if your husband tries taking her out once a month on a special "daddy/daughter" time so that she feels that she's not lacking for special time, and while they're together, he needs to concentrate on being very calm with her and focusing her energies on talking and behaving like an adult as opposed to engaging in her physically playful taunts. (having said this...he also needs to make sure he gives the other kids special time too so that she doesn't feel superior and the other kids don't get their feelings hurt)
Another thing to seriously take into consideration is to make sure that you treat her NO differently from your other children. DO NOT handle her with kid gloves because you feel like she's sensitive about the divorce that happened a decade ago. It happened, you can't change it, and you give her as stable and loving a home as you can....being bitter or sensitive ultimately only hurts her in the long run. Lay down the law when she misbehaves, just like you would with your other kids. If they tried to take away gifts from a birthday child, wouldn't you immediately deal with it, tell them it's unacceptable, and if they don't like it, you separate them and do not let them rejoin any group until their behavior is acceptable AND they must apologize. Self-entitlement and jealousy issues can be a big problem with step-children even though they may not be apparent on the surface, and by indulging or even "making allowances" the main person that suffers is that child.
If she doesn't feel like she's acting out or nothing changes and you are continually sending her away from the group for rude behavior, then try video-taping these episodes, and let her see herself (this can be done non-confrontationally by simply having a "family video night"-or, if need be, flat out call her out....she'll be angry, but you are the parent and you need to do what you need to do to make sure she doesn't learn a crueler lesson out in the real world). Chances are good that without anyone saying anything, she'll be embarrassed and may begin to adjust her behavior by herself.
I would shy away from discussing the "flirting" with both your husband and your step-daughter....that will just be embarrassing and cause problems for everyone. Suggest to your husband that you feel that certain ways of playing aren't appropriate for her anymore as a growing teenager and that you'd like his help to encourage her to behave more maturely in an appropriate way. But! You do need to have his support in dealing with her bad behavior...no matter what, you two need to present a united front. Sorry to have written such a long response but these are all issues I have seen in my own family. BEST OF LUCK!