What is the legal age for a sibling to watch their younger sibling in New Jersey?

Depends on the length of time the 15 year old is watching the children. There is no set legal age to 'babysit', however most YMCA or Red Cross Babysitting courses target 11 to 15 year olds.

Maturity plays a BIG role in watching children, so I'll assume the 15 year old is and knows basic first aid and life saving skills, with the younger siblings knowing how to call 911 when given direction to do so. Just to be on the safe side, I would recommend the 15 year old taking the course and any of the other siblings ages 11 and up taking the same course together.

Finally, three youngsters can be a handful for any adult let along a teenager, and it isn't just watching them that is important. Have a plan in place for disciplining, as in time outs or a "cooling off corner" should things become overwhelming to any of the four children involved. Planning activities for the four to do keeps children occupied until a grown up can resume time with the children.