Okay, something VERY similar happened to me not long ago. In fact in my case I was probably at even less risk.
She probably has nothing to worry about - the chances in terms of statistics that whoever's blood it was was HIV positive is about 1%. And if the person was positive and their blood entered the bloodstream of your sister, it has an extremely low chance of developing into HIV - I believe we were taught less than 1%, but could be wrong.
To sum up, she wouldn't be wasting time if she got a test but it probably isn't necessary, UNLESS she's working in healthcare. If that's the case she might be obliged to get a blood test, as I was.
Hope that helps!