What an awful position you’re in, but from reading the other thread, it seems like your dd is in an even more difficult position. It doesn’t sound as if her df is looking out for her, but rather as if she is trying to look out for him.
I haven’t much to offer - it’s a massive struggle to try and create a home environment that is safe for a neurodiverse dc but also for a neurotypical dc at the same time.
My pair need me at the same time - when ds is struggling and kicking off, he needs my help to regulate, but it’s an anxious time for dd who also wants her mum. Often her needs get put on hold and we take time out together for a chat later. It’s not ideal. I take her out in the car where she knows she has my attention, and we won’t be disturbed or interrupted.
And like your dd, mine has learned to be aware of everyone’s emotional needs, when she’s supposed to just be a child indulging her own. 
One of my realisations has been that I have tended to focus on the battle to get outside support for ds, when dd needs some too. Because it’s so soul crushingly difficult to get support for a dc with severe needs, I’ve been unconsciously assuming that I can’t get help for dd either because her distress isn’t as acute.
I’m going to look for some kind of counselling for her as a priority because I think she needs a safe outlet and to have her experience validated. And her emotional landscape isn’t secondary to her brother’s.
Sorry if this is a patronising question but have you been able to sit down with your dd and hear her perspective on what is happening and what she needs?