It sounds like you've got a great baby sitter and you should make good use of her!
You said yourself that she is quite mature , and being your sister , you'd know her pretty well.
If teenagers and young people are fairly mature they actually make great babysitters , because they have the energy and the fun factor that kids love. I am a lot different now as a Mum then I was a couple of years ago as a babysitter. They have the energy and patience to sit there and read 15 books before bed , because they don't have to think abuot the fact that there is 2 loads of laundry and the dishes to do !
I understand how hard it is to leave our little ones though! Just enjoy yourself though! Mummies are people too !