 Solution

Treat brothers as male individuals sharing at least one recorded parent, including half-brothers. Derive a sibling helper and then select the recorded sex:

siblings :: Database -> Person -> [Person]
siblings db p = unique [q | parent <- parents db p,
                           q <- children db parent, q /= p]

brothers :: Database -> Person -> [Person]
brothers db p = [q | q <- siblings db p, sex q == Male]

The exclusion q /= p prevents a male individual from being his own brother. Duplicate removal is needed because a full brother can be found through both parents. The membership condition isIf the intended convention is full brothers only, replace the shared-parent condition by a common mother and a common father in the recorded certificates. The definition above states the convention explicitly rather than silently discarding half-brothers.