I probably wouldnt have done it,but assuming the parents comforted her later and discussed it with her,I doubt this alone would be a big deal. Sounds like the kid needed a nap and it was kind of cute that she loved her brother so much and didnt know he would grow up. I had mixed feelings.
I saw that video a few days ago but I really didn't think much of it. Children are very emotional creatures, you know. I know when I was a kid (up until I was like 13, honestly) I would break down crying over EVERYTHING. My brother stepped on my shoe: sobbing. Someone was using the computer when I wanted it: sobbing. My sister didn't let me borrow her toys: sobbing. Most people thought I was just being a brat, but truth was I just couldn't control my own emotions yet. The kid was probably tired and in an emotional mood, and the parents probably just thought it was sweet that she loved her brother so much. I wouldn't say a five-year-old girl needs therapy for crying.
I agree with cold war kid. Kids often deal with emotions in very expressive ways. Things that we have learned to take for granted as adults are new and sometimes overwhelming for kids. If she's still doing this when she's fifteen, then it might be a problem.
Kids are especially apt to act this way when tired,overstimulated or worried about something else. Happens all the time. I once had a little girl have a meltdown because she wanted to be a real princess like the little princess she saw in a magazine. She was fine after her nap.