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Please be rest assured that she does not acquire any financial interest in your proeprty, merely by residing there. Indeed, she will not acquire any legal right to remain living in the property, in that all she has is a Licencne to remain living there, for as long as you are happy to do so.
A Licence is whereby a third party moves in and lives with the owner of a property. This is not a Tenancy, which only applies if the third party lives in a property without the owner living there also. Under a Licence, the owner can ask the third party to leave at any time, and it would be normal and reasonable to give the third party 28 days to vacate.
Only if she were to financially contribute to adding value to the property, would she have any financial interest in the property. ie if she paid £10,000 for a Conservatory for example. Even if she were to pay towards the utility bills, she would not acquire any financial interest and nor would she gain a greater legal interest in the property, other than what she has at the moment- namely a Licence. Only if she adds value to the property would she then be able to claim that money back on any subsequent sale.
I hope this assists and sets out the legal position to you.