Before anyone says anything about the answer just being wrong...no.
On'yomi break down:
元 - ゲン/ゴン; 始 - シ, 以 - イ and 姉 is シ.
See where there's something wrong here?
Both 始 and 姉 have "シ" as their ONLY On'yomi.
I took a shot in the dark and guessed the first one.