Just a rant about my sister

So basically my sister has a two and a half year old son. He has minor hearing/speech problems. He was also colic as a baby. But he comprehends and follows directions very well. Anyways, whenever I'm over at her house, she acts like her son is such a chore, I'm talking like she will tell him no very firmly when he isnt doing anything wrong (that I can see?) And when he doesn't listen to her instructions to stop the first time, she will raise her voice and tell him "you better stop". And whenever her son cries, she will threaten him with naptime and act like his behavior is super bad. The problem is, I have two kids myself. My oldest is a year younger than her son, and my second is 6 months old. So I know what a child around her son's age is like. He really isn't a bad child. I've babysat him plenty of times and I even lived with them for two months. I never see him act outrageous. I'm just tired of her treating her son (which she planned) like he is such a hard child when he really isn't. Her husband isn't any better. She will be like "will you please get your son" and he will sigh really loudly and tell him " come over here right now" and then get up after him and tell him "you need to stop. This isn't okay. Knock it off right now". I just don't know why they act like he is a chore and like he is the hardest kid to deal with when he is actually a decently behaved kid??