I want to tell my SIL off for crying at the hospital
Why do you want to tell her off for?
Because she keeps on complaining that she is sad and it's so annoying
It is understandable to be annoyed at your sister-in-law if she keeps complaining, but you should not tell her off. Considering that she is in a hospital, you should talk to her and find out why she is sad.
Why should I listen to a stranger online? Maybe it's a prank.
I sincerely doubt that your sister in law is pulling an elaborate prank on you. You need to find out why she is in the hospital, and why this is such a sad time for her. Her complaining could be justified if she is about to lose a loved one.