Judy,

I use to babysit when I was 16-18 y.o for a diabetic little girl...well she was 12-14y.o so I guess it was teen sit :7)  anyways...I never sat for her when she was sick, that was just a given that the care required for her was above my level.  On well days, she knew her routine and all I had to do was make sure she followed it.  Make sure she checked her sugars & took her insulin...her parents had it all written out if I had any questions.  Also, if she felt even the slightest bit "off"  we were to call her parents and go from there.  \

I think you should (when you are ready for her to be in charge of her younger sibling) sit down with both kids and go over everything.  Better to have them roll their eyes and say "we KNOW Mom!!" then miss something.

Jess