Autistic 9 year old hurt me in meltdown

People tell me all the time 'he looks normal to me' 

Fantastic! At school he is highly intelligent and mostly well behaved and comes home tired goes to his room and we only have the odd outburst or meltdown.

First 2 weeks of the summer holidays were long and hard while he adjusted to lack of routine with and time with his younger brother (Autistic and cerebral palsy) and sister. First violent outburst ever(1 week into holiday) hit sister continuously even when she was screaming and took 45 mins to calm down. Second outburst I got hit once whilst protecting children and my mother who has had a stroke and very disabled lasted 6hours but was mostly verbal and self hurt. Today 3rd outburst and by far the most violent. It lasted 2 hours at least he hit both children very hard, hit doors, shot nerf bullets at us. I took him upstairs to calm him where he hit me over head and back repeatedly and tried to kick and push me down stairs and bit me. He then trashed the lounge and ran to kitchen and threw water and clothes everywhere, room trashed. He then hit me over arm and legs with hard end of lightsaber toy. I have bruises.

How do I deal with it....he's very sorry after but that doesn't excuse it and now my youngest is starting to hit and my daughter is being very rude to me. Non supportive husband too.

What do I do what help is available?