As Pp said , you need legal advice so you know the options. Then you need a discussion with your sister.
you may also need other advice , eg an accountant or a letting agent.
For example , Pp have said “ you might get £500 / month for rent “. But you will have to pay half of all repairs and maintenance , insurance etc . And you will have to pay tax on any profit you make.
£6,000 a year might sounds a lot, but costs will be high. If your sister wants a new kitchen, how will you decide if the budget is £5k or £10k?
What is she wants the place redecorated but you think it’s fine ?
Do you know the legal duties that you would have as a landlord ? Do things like annual gas safety checks, PAT tests, EICR apply if your tenant is also a part owner ? Whose job will it be to deal with these things? Who will pay for the legal documents that will be required ?
what will you do if your Sister stops paying rent ? Perhaps for a genuine reason like she loses her job?
What if she stops paying because you refuse to replace her carpets or the gas bills are too high ? Tenants stop paying for reasons like this all the time.
Do you really want to go down that route ? Because if your sister was going to be reasonable , you wouldn't be where you are today .
People think that being a landlord is money for old rope but it can be a nightmare, especially with difficult tenants. So you need to information so you can consider each option carefully before you offer it to her.
In your situation I would want it sold , either to your sister ( at market value) or to a third party.