Volker Kauder, 63, is demanding that his younger brother Siegfried, 62, be expelled after he defied the party leadership and said he would run against the CDU in September's election.
Siegfried declared he would stand as an independent after failing to get his party's nomination in his current constituency of Villingen-Schwenningen in Baden-Württemberg.
Now, his older brother - and more senior party member says there is no alternative but to kick him out of the party altogether.
“That's a classic case for party expulsion and that is also what has to happen,” the CDU politician told Schwäbische Zeitung on Wednesday.
“He is running against one of the CDU's candidates and wants to remain a party member at the same time. Independent of family ties I have to say it straight: that's not ok.” he said in a separate interview with the Saarbrücker Zeitung.
Siegfried, who also currently heads the parliamentary committee on legal affairs, said he would not fight to stay in the party if he were asked to leave.
“If there's an expulsion procedure I will of course submit to that,” he told DPA news wire on Monday.