14 year old babysitting?

So my 14 year old daughter is extremely mature for her age and I trust her to be home alone while I run errands, she recently started asking if she could watch her 3 year old brother while I run quick errands so she can earn some extra money. While I know I can trust her to watch him while I run to the store it still makes me a bit nervous because my son has always been by my side 24/7. She tells me all her friends get to watch their siblings for money and it’s not fair that she doesn’t get to. I’m just so used to taking him everywhere with me, does anyone see anything wrong with a 14 year old watching their sibling say while I run to target and such?