Upvote:3
The 'family visitor' programme was abolished in April 2015. Instead you can apply for the Visitor (standard) Visa.
For your questions...
He should include all the things you mentioned plus bank statements demonstrating his capacity to sponsor you along with the landlord's permission that you can stay in the flat for 3 weeks;
You should submit your own bank account statements. Your parents' statements may not be considered if they are not UK residents.
Yes, your BIL should submit his employment contract and salary slips to demonstrate his sponsorship capacity and that his T2 is still valid.
No, they do not care about flight tickets.
In general, your BIL acting as economic sponsor should submit everything listed in the guidance that would normally be submitted by the applicant. Note that they will be reluctant to accredit anything submitted by sponsors who are not UK residents.
NOTE: if your BIL's tenancy agreement specifies that visitors staying for 3 weeks are ok, the landlord's permission is not required, but he should submit the tenancy agreement instead.