Jacob Israel Mirkes - Is Son Not Brother

According to the Smorgon YB, he is the son of Shlomo Zalmen Mirkes not the brother:

"These two books are adorned with approbations from the Torah giants of Lithuania and famous rabbis from other places, including Rabbi Avraham Katzenelboigen, the head of the rabbinical court of Brisk of Lita [Brest Litovsk] and Rabbi Yisrael Mirkin, the head of the rabbinical court of Minsk (grandson of the Gaon Rabbi Chaim HaCohen, the rabbi of the district of Smorgon)." Smorgon YB p81
Shlomo Zalman Mirkis, A.B.D. Mir