Ohh my goodness - you are living out one of my biggest fears at the moment. I am SO sorry to hear this happened to you. I am really glad to hear about the step-father though - has your SO talked to him about things? Or are you comfortable or even wanting to talk about things with him at some point? To at least give him the information on Your terms, to try to help give you some control back into the situation.
Recently I had a faculty member drop a hint about my condition in front of a fellow classmate of mine - I was mortified beyond all belief. The fact that I'm Bipolar II is something I keep very quiet, especially within my college, however this woman must have come across some papers of mine for filing. She's apparently not very aware of what 'CONFIDENTIAL' means, because like I said, in front of a fellow classmate she used the words: you, bipolar, meeting, and accommodate. Despite my mood fluctuations, I swear I have never once before that moment thought about slappin' a bitch on the faculty staff. How could she just say things like that in front of another classmate? I was appalled. Mercifully the classmate was my closest friend and is in the loop, but truthfully I don't know what I would have done if it was anyone else.
Personally, I guess I'd ask my SO why he had never told me about his other family members before now. That may have made me more comfortable with things, or even tell the family that i'm struggling with personal things - anything, to help ease into it. I can't imagine how you must be feeling right now :/. My advice sucks apparently, I don't know if I could even call that advice... I'm sorry!
Pep talk time: You're amazing. Our Bipolar II condition feels like it rules us most of our life but it isn't us. You're awesome and stand alone from your condition. You aren't a condition. You just have a condition - very different. You're incredibly fuckin strong for not having ripped your SOs head off with your bare hands (probably would have been my first coping mechanism...). I don't know how close you are with your SOs family, or how close he is with them - but in my mind, if this changes how they view you and him, they can kindly burn in hell. If they have known people who also struggled and lived with this, then they should even PARTIALLY understand the struggles you face on a daily basis. (The struggles suck, but we don't talk about them, we just accept them, right?). I hope if the family does change how they view and treat you, your SO will stand up for you and say something. It wasn't fair for him to say that without you being there or even telling him it's okay to let them know. But I hope that if the situation arises he will support you and stand beside you. You and him against the world - right? Make it through another day, that's all we can hope for. Like I said - you're awesome. I don't know you, but for someone with a like condition, I feel like maybe I can relate to how you feel. Keep going. Let me know if you want to chat! (I mean my advice is awful but you know, the offer is always there) :)