i think it's VERY dependent on the child - as a 12year old, i was looking after other peoples kidlets at least once a week - but i was a very mature 12 year old!

your stepson doesn't sound at all mature enough to handle being on his own - esp in YOUR home which isn't his primary home! you obviously are aware of what the issues are with him staying at your home, and it is up to YOU to decide whether ANYONE is left alone in your home

your MIL sounds like she was trying to manipulate your DH into making you feel bad about standing up to her - stuff her! she did the wrong thing and needs to be pulled up on it! you're not at all negative - you're realistic about your living situation - you're MIL just needs to learn that not everyone does things the same - each family, each home, each individual - is different!