Baseball tends to be a family business, and the League includes a number of brothers:

Jesse Barnes, P, Cincinnati
Virgil Barnes, P, St. Louis

Jimmy Cooney, SS, San Francisco
Johnny Cooney, PH, Cincinnati (minors)

Bibb Falk, OF, San Diego
Chet Falk, P, Texas
[A look into the crystal ball:  Bibb Falk is the Falk in Disch-Falk Field at the University of Texas, where he will coach for many years after his playing career.]

Bob Meusel, OF, Boston
Irish Meusel, OF, Los Angeles

Jake Miller, P, St. Louis
Russ Miller, P, San Diego (minors)

Oscar Roettger, OF, San Francisco (minors)
Wally Roettger, OF, Montreal (minors)

Bobby Schang, C, Detroit (minors)
Wally Schang, C, St. Louis

Joe Sewell, SS, Boston
Luke Sewell, C, Chicago
Tommy Sewell, PH, Boston (minors)

Lloyd Waner, OF, Cincinnati
Paul Waner, OF, Detroit

This list includes only brothers currently on a 40-man roster in the League.  There are even more players with brothers who played previously or are likely to play in the League in the future.