Is it permissible to request an autopsy of the deceased brother's body two years after his death to ascertain the cause of death, especially given suspicions about his wife, and out of consideration for the mother's feelings?

We ask Allah to have mercy on your brother. We advise you to be patient and to say "Inna Lillahi wa inna ilayhi raji'un" (Indeed, to Allah we belong and to Him we shall return). Accusations of murder should not be built solely upon mere threats or a sudden death when one was in good health. The default assumption for a Muslim is innocence. It is advisable to refrain from pursuing this matter unless there are strong indications. The judiciary is the one that determines the acceptance of doubts and indications and authorizes the necessary procedures.