Upvote:0
I assume you mean what she should write in her letter of sponsorship, which will become part of the visa application.
The letter should reflect what she is willing to sponsor
The reader should, as a result, understand what she is willing to sponsor and what not
Upvote:2
In addition to telling what she will be providing you with, you must document that your sister is legally resident in the UK.
Also, since she lives in rented accommodation, she must show that her landlord is okay with her boarding guests -- either in the form of a lease that explicitly allows this in general, or a letter from the landlord that permits it.
It's also a good idea to document your relationship, e.g. with birth certificates showing you have the same parents.