It depends. Unless you have proof of after-tax money that funded the plan, most pensions are taxable to the beneficiaries.
Pension income is exempt only to the extent of after-tax money was used to fund them. Traditional individual retirement arrangements (IRAs) that may have been established could have both pre-tax and after-tax contributions. The 401(k) plans at work are not tax exempt because the funds placed in the plans are either pre-tax employee funds or employer provided payments.
Since your mother was from Georgia (GA) there may not be any Indian Tribal relevance, a link is posted for your review.
The Form 1099-R should be received by you and your brother from the pensions where you were the beneficiary. The plan administrator will enter the taxable amount or they will check that the taxable amount has not been determined. If you have no other information, then they would be fully taxable.