How does she handle being alone?
The bad memory following an outburst really can feel like whiplash. I think it's Shari Manning's book Loving Someone with BPD that describes the physiological link between memory and rage.
I found the site AspergerExperts.com to be helpful in thinking about the social skills aspect. My H and I have three kids who are various places on the spectrum (S18, SS20, SD22) and they all have issues with hygiene, appearance, social norms.
SD22 doesn't seem to feel food on her face

and all three kids don't seem to be aware of what their clothing is doing. SS20 has the most pronounced focus on a special interest, although SD22 can talk for hours, too, if people allow. Oddly, you can most see ASD traits in pictures. She doesn't smile, or if she does, it's more like she's forming the shape of a smile if that makes sense. And she'll try to joke about the same thing over and over and over and over again, not recognizing that it wasn't really funny the first time, and didn't get better the second or third or fourth time. She's also not aware of personal space like you say your SD12 is. That quality, plus her inability to be alone, has been the most confounding for me as her step mom. Her hugging is pretty aggressive, too.
How often is SD12 having meltdowns?