Violent behaviour

I’m mum to a 12 year old diagnosed with asd when she was 9.. although the paediatrician seemed reluctant and said it was mild. We were diagnosed and deserted. Sadly we are struggling hugely with her behaviour. She is the master of masking and so when at home is exhausted. As a result she sits in her room for hours on end on her phone or watching tv.. I do understand why this is but life for the rest of the family stops. She won’t allow us to speak to her or ask her anything and if we do it results in a meltdown. This escalates very quickly and ends now in her hitting or kicking us. She threatens to kill is or kill herself.. as much as these words are awful I’m certain she doesn’t mean it. I have no idea how to parent her.. we have a younger daughter who is hugely affected by what she sees and hears and things just seem to be gettting worse. 

thanks for any advice