My opinion?
Of course you realize your sister is horrible mean person. Her entire response is me oriented and cold and cruel. She is psychotic.
Second, you must already realize this and you should quit being surprised by it and expect it from her.
Third, because of the above, why the hell would you or your husband feel the need to explain yourself to a crack pot? This bitch doesn't care about anyone's feelings. You can only go home and thank your lucky star that the blood from your normal parent diluted this potential personality trait in you.
Forth, you never mentioned how old or what is wrong with your son. It's hard to figure out if he is overly sensitive, but I am guessing yes. However, that is much better than being a psycho.
Fifth, the real issue to be dealt with is explaining to your son that assholes exist and yes, even in our own families and how to deal with it in a more constructive way than crying. He needs to learn to express how he feels with his words, that will empower him.