If my brother was captured, can they give me a reprieve from the army?

No, this reason is not provided for in Art. 23 of the Law of Ukraine "On Mobilization Training and Mobilization". It is possible to receive a deferral only if: women and men whose close relatives (husband, wife, son, daughter, father, mother, grandfather, grandmother or biological (consanguineous, non-consanguineous) brother or sister) died or went missing during the implementation of security measures of national security and defense, repelling and deterring armed aggression of the Russian Federation in the Donetsk and Luhansk regions, as well as during the provision of national security and defense, repelling and deterring armed aggression against Ukraine during martial law.