Ко
Костя
My older brother asked me, what I was doing today.
Не уверена. Примерно.
My elder brother ask me,What did I do today?
У вас не дано слово, вводящее прямую речь, от которого очень зависит грамматика! Если say, то My elder brother SAYS what I have done today. Если said,то My elder brother said what I HAD done THAT DAY.