Do special needs people know when people are mean to them?

I'm 18 and my nephew is 14. I have a sister who's 36 and for the longest time I was raised with him. When he was 10 and I was 14 my sister came back for him with her boyfriend who's she's now married too. My nephew is severely special needs. He has autism and only knows a few words. My mom tried to keep my nephew but my sister took her to court and got custody. The last 4 almost 4 years my sister and her husband are so mean to him. They laugh and him and call him retarded. They yell at him all the time. During Thanksgiving he was finger picking his food and my sister told him to stop and her husband said "Fucking eat it. Dumbass." My family has called my sister out for it and she just laughs and says "He doesn't understand us." But I feel like he does because he's so uncomfortable around them. Especially my sisters husband. He's the meanest to him and my sister lets him and joins in herself. He always is overwhelmed and uncomfortable and they just yell at him and call him names and my sister says he's a retard and doesn't understand. But I feel like he does. Maybe I'm wrong but it seems he understands what they say to him.

Edit: I don't understand why people are coming at me. First off I'm 18 and literally still in highschool. I have no idea how that stuff with CPS works. Second of all my mom has been the one handling all that