I forgot to say that for ASD children, sensitivity to food can be not just a question of "bad" behaviour. It is a bit like asking a NT person to eat something they consider absolutely horrid like tripe or beetroot, or pig's liver to get an ASD child to even touch something with the wrong texture or ingredients.
People have said to me that the mere smell of something can make their ASD child physically sick. I think you have to take this into account with her perceptions of mealtimes.
Also if she has been used to being with her mum at w/e and that has ceased she may be reacting massively to the change in her life/carer. I think ASD children are acutely sensitive to whether people love/like them or not, even when they are not on the surface likeable/loveable.
It is so hard for you, even with a child I loved very much, my dd, I could go to the white knuckled stage very quickly. I suppose I really had examine why she was reacting to situations rather than holding her to account for her bad behaviour all the time. With my ASD child I sort of knew all along that he needed x and y to manage (even before he was diagnosed) so made lots of allowances that I never gave dd.
I think the whole "Bonding" malarkey cannot be overemphasised, especially with ASD. I think once they know you are on their side, unconditionally, and not going to shout, they begin to find it easier to a)be calm b) express their needs. When you tell an ASD child off angrily, they usually can't take your emotions as well as their own.
Ds2 can behave apallingly, and dh shouts at him, and it often makes things much much worse. Whereas having a place to calm down, and a quiet boundary setting helps much more. "When you shout/hit, you have to leave the room and sit here instead" That works very quickly for strops. Boundaries, but unconditional love continuing.
I've thought of another tip: what about small meals at small intervals, and with one other person her own age, a sort of buddying up, rather than big meals three times a day? Again, it seems like special treatment but might make a difference to her perception of food. I'm sure if I was a 13 year old sitting with a TA on my own, watching how much I was eating, like a hawk, I would probably not feel particularily hungry either
just leftout and grumpy.
I forgot to mention that dd was always fine in other people's houses and at school, even when she never ate in ours, it was a pyschological thing to do with family meal times, so not quite relevant to an ASD situation in some respects. For example she loves school dinners, and big canteens.