It sounds as though they are all seeing your son through his behaviours only. Easily done when he plays up specifically aimed at his relatives. However. His relatives are grown ups and your son is the child. ASD is incredibly complex. One of the often key things is emotions. Emotions can feel far more extreme to someone with ASD, especially a young lad of 12 on his way towards puberty. He's rapidly leaving childhood and moving towards adolescence. Most neurotypical children find this hard.
Is there anyway, you could suggest that your dh and key relatives could find a local autism course to attend. I know this may sound daft, but maybe, it could change their opinion of your son if they achieved a better understanding of ASD. They could actually become a really good support to you too.
At the moment, son plays up, they attack, you defend. I assure you, children with ASD can have extremely good intuition, and can pick up on weather people like them or not. My DS is 4.5 and has taken an instant dislike to my mother. (I don't blame him one bit....secretly, she's a witch :)) Whenever shes on a video call to us, he walks up to the camera and says really assertively, shut up you stupid BxTcH. He's actually only met her once for 2 hours, when he was 3 and became really withdrawn and refused to have anything to do with her.
Your ds sounds hurt. He doesn't know how to express this, or cope with it, and so he's lashing out.
Iv'e not tried story boards, as my ds avoids everything which isnt on his terms, but maybe you could try something along these lines with your son so he gets a chance to express himself to you. Once you get a little more insight from his perspective, you will be able to gently and slowly work with him to manage these situations, and avoid melt downs. He has a voice and needs to be heard. You know he's not the boy they are seeing and branding him as. Relationships take work from both sides. This cannot all be on your ds shoulders.
If the relatives and your dh wont attend a course, then at least suggest that they read a book. There are some really great ones out there. Some written by young adults, about having ASD and their experiences through childhood. Previous generations can have a dated understanding of ASD. They just think 'autism' and struggle to see the child. Unless the child is flapping, rocking and non responsive, it's hard for some people to understand how it effects people. If the child looks and behaves pretty normally, then people expect normal behaviour.
Be proud of yourself, because you are doing everything right. The first thing I was told to do and have been told countless times by some really good professionals, is to nurture. Nobody is ever going to know exactly how your son is. No two people with ASD are the same. Anxiety and depression can factor highly with ASD because of a whole variety of hidden triggers, which don't factor with most of us. Good news is, from what I've read, most children with ASD actually settle down during their teens. Maybe because all theirs peers become so focused on who they each are and how they look etc, so for the first time, they all have something in common. children with ASD are often highly aware of their differences from a very early age, and so by the time puberty kicks in, they already have a good balance of who they are.
Ref your dh, it's great your dh is making an effort. Sometimes with ASD, the child can focus more on one person. In your case, this is you. Your'e his safe place. You understand him. Your dh may never have the same position as you, but he will have a place. They just need to find where that place is. Maybe set up something your ds loves doing, then feign feeling ill, so your dh has to step in as a hero, and take your ds out, leaving your dd to go stay with you, or see her relatives.