You don't need both your brother and dad. Choose 1. Make life simple.
Since all file jointly, there will be 2 I-864 as well as 2 I-864A.
If you do both dad and brother, the above becomes 3 each. More paperwork, which you should try avoiding.
You need IRS transcripts. Download them easily from the website. If you send 1040, it'll be 1040 + paystubs, which is more papers. Take easy way out.