6 months family visitor visa

[Mother101]

I am an Albanian citizen helping my mother to apply for a short-term family visit visa (6 months) so she can go and visit my sister in London. She recently moved to UK with a 2 year settlement visa (joining her husband, a British citizen). I have a few questions about the application:

1. Is there any specific requirement about how much many you need to have in order to qualify? (For example a certain amount of money per day)
2. My father will be covering the expenses because my mother is retired. Does he need to provide payslips and bank statements? How do you prove that she is retired?
3. My sister and my brother-in-law will accommodate my mother during her journey. Do they need to provide payslips and bank statements from the last six months as well? Do they need to provide simple scanned coloured copies of their passports or copies legalised by a notary? Also do they need to provide utility slips, tenancy agreement?