So ultimately do you think it's more or less reponsible to replace an 80 yo babysitter with a 16 yo?

For an 8 year old who is very mature and independent for her age. It seemed that my MIL was having my daughter watch her mother under the guise of the her mother watching my daughter. The girl's mother will be most of the time, but at 16, I think the girl can adequately handle a child in her care for 10 hours a day.