I mean, nobody thinks she lives there too. It's in the first bloody sentence... shes lived in my house since we moved to a bigger property.
Well that's exactly what a lot of posters and myself did think on first reading. I misunderstood 'shes lived in my house since we moved to a bigger property' to mean 'when we sold our smaller property and bought a bigger property my sister came to live with me in the new bigger house'. On rereading, I realise the OPs meaning, but given that lots of people made the mistake it's a bit goady to assert that 'nobody' could be misunderstanding.
I suspect it's because the idea of being able to buy a bigger house without selling the old one is so beyond most of our financial realities didn't occur to us!
As far as what rate of rent to charge, it depends on how you feel about having your sister in the house. If having your sister there if an advantage you (does she keep the place well, does she pay for repairs? etc) then charge less than market rate incetivise her to stay. If you'd be equally happy with a random stranger (who might trash the place, and who would expect you as landlord to keep up maintenance, pay for things that need replacing etc), then charge market rent, and don't be surprised if she moves house. If you are pissed off with her and want her out, charge more than market rent....
Personally I'd much rather have someone I know in a house I was keeping for my kids one day than rent it to a succession of strangers.
I get that people are annoyed on your behalf about the rent your sister hasn't been paying, but you can't really wait seven years then chase the money up(!), so what's happened historically has to stand really.