I'm worried about my sister's husband.
What is concerning you about him?
His cough is so heavy and he can't sleep at night. Plus she treats him like garbage most of the time.
It's good that you feel empathetic towards your sister's husband, especially since he's sick. You should talk to your sister about being kinder towards him and let her know that she should treat him better.
Yeah, I don't know if I should involve myself.
I wouldn't create drama, but letting your sister know her actions are wrong could help remedy the situation. She might not even be aware of her behavior and you could help her improve.