13a-c in that part is asking for what's on their tax return, so just write the total income from the brother and his wife in those boxes. My parents also filed jointly and I had them write in their household income in those boxes and then had them show their individual income (in this case, it was just spilt 50/50) on part 6 question 5 and question 6c.
It's all in the wording. 13a-c asks for "My total income" and then above that for questions 5 and 6c it asks for "current individual income"
PS - if your husband's brother needs to combine his income with his wife's to meet the threshold then his wife will need to submit an I-864a "contract between sponsor and household member" to accompany her husband's I-864.
