I think you need to have a frank talk with your son about what he can say and what he should say. If he is NT, there is no reason he cannot police his words and be kinder. He is older, almost an adult, so the largest share of the responsibility of not hurting his sister is on him.
That doesn't mean you cannot help your daughter to learn not to take the things he says to heart. And if it helps her, can you think of some examples of what he was like at that age when you correct him?
My children are not as old as yours, my DSS is 9 and my DD is 2. but when dd was in diapers, he used to comment about how disgusting it was that she wore a diaper. Of course, she was too young to internalize it, but we did tell him that he was in diapers until he was 4, so he had no reason to call her disgusting when he experienced that phase of his life, as well. Hearing you defend her, and remind him that he was not always so perfect is a growth opportunity for them both... he learns a little bit about looking in a mirror before you criticize, and she learns that the big brother that she idolizes, but hurts her, went through the challenges that she did.
