Other would be the Curry brothers (I think) and the Marquez brothers. I'm guessing they aren't the answer but fun to think of anyway
BINGO ! Yes Monte Attell And the great and infamous Abe Attell were the first brothers to hold titles at the same time. Abe Attell won the featherweight title in 1901, and Monte Attell won the bantam title in 1909 from Frankie Neil...Good show B...
Close, but no cigar E. Joe Dundee won WW title 1927, lost title to Jackie Fields in 1929. Younger brother Vince Dundee won MW title from Brouillard in 1933, lost title to Teddy Yarosz in 1934. Not at the same times...