Helping bipolar daughter

Hi all,
I'm new here and at a complete loss with my 20 year old step daughter. She is undiagnosed with bipolar but my husband and I are certain she has it. There is a family history of it (my sister in law suffers from it) and she shows all the signs of swinging from highs to lows.

She's currently swinging to her typical fall funk. It gets BAD. Last Christmas, she ended up punching the wall, my husband, and fleeing the house, abandoning her 1 year old son. She came back later that night and refused to return for 3 months. It broke my husband's heart to be refused access to his grandson. Speaking of which, my husband is primary caregiver. He spends all his time with the baby, who's now 2. She ignores him unless someone is around to see her being mom. she loves the attention friends and strangers give her for being mom. She can't be bothered with her son otherwise.

Christmas was not the first time she hit a family member. Nor, I fear, the last. She's showing all the "attitude" and quick to fight with us. She flipped on me because I asked her to empty the bagless vacuum the other day. My "punishment" for making her do a simple chore was to leave used tampons unwrapped in the bathroom. She gets very passive agressive like that, letting me know her disproval for something I've said by stealing somethign of mine or leaving disgusting things out for me to find.

The tension is killing me, killing my husband. And her son is suffering because he feels the tension.

She has random sexual partners. One will stay a few months, then leave once they catch onto her mood swings. She's had too many STDs to mention and has irregular pap-smears because of it. The current one claims to want to marry her and adopt her son... they've been dating about a month! I would like to talk with him about possibly getting him to talk with her, but I'm afraid she'll ignore him and break up because of it. She's done that in the past too. One guy was actually a greast guy who would have helped her a lot. She dumped him as soon as he agreed with us about needing help.

I talked to her doctor and explained everything and the doctor was floored.... she tried talking with my step daughter and her advice was ignored. My SD honestly feels she has no problem. She wears her problem is a mean step mom who points out her flaws (like, how mean of me to suggest 4AM is a bad time to come home, or she shouldn't be out drinking at 20 with a baby at home, or how boyfriends shouldn't sleep over every night, or that she should respect herself!) She swears her life is miserable because I hate her and poisoned her father against her. It KILLS me that she hates me so much.

What can we do for her? She needs help. WE need help. What can I do to make the tension ease? What can we do to convince her she'd be happier if she got the help she desperately needs? I can't imagine going through life so unhappy and lost. I myself battle with depression, and have recently gone through a bad spell myself, complete with a bad reaction to Cymbalta (evil drug!!!) so I know how low a person can go, and how much better I feel when I have help.

I appologise for being long-winded. Thank you for reading this if you've gotten this far.