Inviting parents in law to the UK

[vsmartybrain]

Hi,

I am planning to invite my parents in law (wife's parents) to UK, and I am unclear about certain documents that I will have to send across to sponsor them.
1. I am married and live in a 3 bedroom flat along with my brothers family(his wife & child)
2. The tenancy agreement is in my brothers name, though we share rent amongst ourselves.
3. I have a steady job, and have indefinite leave to remain in the country.
4. The issue is , I do not have the rental documents to send across nor utility bills that feature my name. Though the bank statements and the salary slips have the address on it. Is it sufficient to send a sponsor letter stating my current address along with the bank statements and the salary slips.
5.Also how much funds should I show to fully sponsor them for a month?

If any one has experienced the same situation please advice/suggest an alternative.