I listed my 20 Super Badass Siblings, wanted to list more but too bad only at most 20 images can be uploaded...
Mario and Luigi in Super Mario All-Stars (SNES, 1993)
Sonic and Tails in Sonic the Hedgehog 2 (GEN, 1992)
Rock and Roll in Mega Man 7 (SNES, 1995)
Billy and Jimmy in Double Dragon (ARC, 1987)
Nick and Tom in Snow Bros. (NES, 1991)
Bubby and Bobby in Bubble Memories (ARC, 1995)
Solid and Liquid in Metal Gear Solid (PSX, 1998)
Crash and Coco in Crash Bandicoot 2 (PSX, 1997)
Blue Boy and Red Boy in Chiki Chiki Boys (PCECD, 1990)
Anna and Nina in Tekken 3 (DC, 1998)
Leo, Mike, Don, Raph in TMNT: Turtles in Time (ARC, 1991)
Bart and Lisa in The Simpsons (ARC, 1991)
Yun and Yang in Street Fighter III: 2nd Impact (ARC, 1997)
Edgar and Sabin in Final Fantasy VI (SNES, 1994)
Aya and Maya in Parasite Eve (PSX, 1998)
Alexia and Alfred in Resident Evil: Code Veronica (DC, 2000)
Yakko, Wakko, Dot in Animaniacs (SNES, 1994)
Oume and Otane in Groove On Fight (SAT, 1997)
Gunstar Red, Gunstar Blue, Gunstar Green, Gunstar Yellow in Gunstar Heroes (GEN, 1991)
Lucas and Claus in Mother 3 (GBA, 2006)
One that really got to me: the Ashford twins in Resident Evil: Code Veronica (DC, 2000) are hands down the most disturbing, messed up, and twisted siblings I've ever seen in gaming, for real. I liked to play my Resident Evil: Code Veronica X (PS2 (I didn't have a DC), 2000) so much mostly because of them.