Hmm, it sounds as if you answered your own question. Yes, each kid matures individually and differently. 11 IS young for many. IF this girl could not handle an emergency, remaining calm and collected enough to summon help and maintain safety, then she has NO business watching children at all. Also, if she does not have CPR training or baby sitting classes, she should NOT be watching a younger child.
Only In my opinion, I realize, most kids of 11 are TOO YOUNG to assume responsiblity for younger children independently. Some can barely care for themselves. Some can do this, but they are a rare few who I consider mature and capable enough to safely handle it. I guess I am just an overly-cautious mom. I just NOW allow my 12 year old son to watch my daughter (5) for 30 minutes or so while I run errands within 10 minutes of home. And, it was not legal before this, that I know of.
But he knows CPR, we have neighbors all around to whom he can turn day and night for help, and he IS calm and cool enough to know what to do in emergencies. We have drillled the fire, water pipe blow-out, earthquake emergencies already. But that is only 30 MINUTES, not hours, I leave them alone!!!! And never at night, only in daylight hours.
In your niece's case, I think too much responsiblity is being placed on her too soon, based on what you say here.