my sister has lived with me for the last couple of years i am finding it hard finically i need a figure to show her but how much rent should i charge her she does not put anything into the house
A fair way would be add all the costs of running the house - gas, electric, water, council tax, telephone, tv licence, rent or mortgage, household insurance, and divide it betwen all the people in the people in the house.
Then add on the extras - food, cleaning/bathroom materials and so on and decide a fair rate for that.
Presumably she has her own room and shared use of kitchen bathroom lounge etc. You might be able to find ads. in local papers, shop windows etc. from people wanting to let rooms out commercially on this sort of basis. This could give you a guide, as an alternative approach to Ethel's or to double check the result.
Don't forget that if your sister does anything in the house like preparing meals for you or shopping for you or cleaning (other than her own room) you need to allow a reasonable amount for this.
There is another issue you need to consider. Whether or not you are collecting rent from your sister, the Inland Revenue will calculate that you have received a fair rent from her and will tax YOU as having received that money. I'd get her paying asap. Lots of people are now getting caught in this trap as the IR are increasingly clamping down on relatives providing FREE accommodation - as they see it - at the taxpayers expense.