Is what befell me in my studies a punishment for revealing my sister's secret, which I swore to keep?

You should not have disclosed your niece’s secret to anyone, especially after swearing to your brother to keep it. We ask Allah for forgiveness for what happened due to forgetfulness. You must seek forgiveness from your brother and his daughter in general terms, without informing them of the truth of the matter, to avoid their anger. The questioner must offer expiation for breaking an oath.