I babysat at age 12-13.
The only thing that would concern me is why does your 18 month old need an apnea monitor? Is there an issue or are you just being a bit overprotective? If there is an issue, I might think twice about a 12 year old handling it, more because if something did happen, how would that affect the 12 year old? Would she consider herself at fault, even if she wasn't at fault and did all the right things?
As far as your mother...I keep mine on a "need to know" basis. There are some things about my life that she doesn't need to know, so I don't tell her.