Home Alone - Age 10 and 13?

This summer my brother and his ex-wife are planning to leave their kids home alone while school is out. The 13-year old is not particularly mature nor “motherly/fatherly” toward his brother (nothing out of the ordinary, though). I think this is way too young! I was thinking about helping to pay for some daycare or summer activity, but was wondering if I am off base in terms of them being too young to stay at home.