In the sense that they are family and the older children are 7-9 themselves and used to helping out with the younger ones I'm sure I don't see a problem with it. Large families develop routines and expectations and it works very well most of the time.

We try to mix programming and cooking and outings in with our babysitting and that is where we get confused on the amount of time and effort it takes. Lining a group up in front of a video, offering the food mom left ready to be microwaved and only having 2-3 of the group of 8 still in diapers is a totally different scenerio. A 17 year old with lots of experience with kids obviously from years of practice will be good at it too. He likely has had responsibilities for younger siblings and cousins since he was 5 so 10-12 years worth. Really don't see an issue at all. Large families do what they have to do.