There have been many brother tandems to compete together in the UFC.
Brother Teams to compete in the UFC
1) Matt and Mark Hughes, behind the barn
2) Nate and Nick Diaz
3) Rogerio (little) and Rodrigo (big) Nogueira
4) Frank and Ken Shamrock (adopted but fine)
5) Anthony and Sergio Pettis
6) Jim and Dan Miller
7) Joe and Dan Lauzon
8) Matt and Nick Serra
9) Clay and Jason Guida (Jason was on TUF 8)
10) Jake and Jared Rosholt
11) Erick and Augusto Montano
12) Junior and Raphael Assuncao
13) Iuri/Yuri and Ildemar Alcantara
14) Junie and Rob Browning (Rob was on TUF 9)
15) Rashad and Lance Evans (Lance was on TUF 8)
16) Jake and Joe Ellenberger
17) Chris and Brian Camozzi
18) Yuri and Danillo Villefort
19) David and Daniel Teymur
Brother/Sister in the UFC
1) Naoki and Mizuki Inoue
Rolles and Roger Gracie - are they brothers? How are they related?
Brother teams in MMA but not the UFC
Yasuhiro and Toshimichi Akagi
Karl and Bertrand Amoussou
Chase and Carson Beebe (Carson competed in WSOF)
Gilbert and Herbert Burns (Herbert competes in One)
Fedor and Aleksander Emelianenko
Caros and Ben Fodor (Phoenix Jones) are adopted brothers
Patricky and Patricio Friere
Pat and Ryan Healy (Ryan competed in Bellator and others)
Jeremy Horn and Matt Anderson are half-brothers
Enson and Egan Inoue
Erik and Keoni Koch
Naoyuki and Hiroki Kotani (Hiroki competed in Shooto)
Chuck and Sean Liddell
Douglas and Dhiego Lima (Douglas competes in Bellator)
Lyoto and Chinzo Machida (Chinzo competes in Bellator)
Michael and Brad McDonald
Cole and Micah Miller (Micah competed in WEC but not UFC)
Gegard and Gewik Mousasi
Takaharu and Takehiro Murahama
Anthony and Chidi Njokuani (Chidi competes in Bellator)
Alexandre Franca and Leonardo Nogueira
Khabib and Abubakar Nurmagomedov (cousins, and adopted brother)
Alistair and Valentijn Overeem
Rousimar and Rick Palhares
BJ and Reagan Penn
Mauricio and Murilo Rua (Murilo was only in Pride)
Chris and Andreas Spang (Andreas competed in Bellator)
Marcus and Johan Vanttinen
Cousins in MMA:
Oliksiy Oleynik and Alexander Butenko
Royce, Renzo and Roger Gracie (and then you can go through the tree to find more)
Pat and Jeff Curran
Brother/Sister teams in MMA but not the UFC:
Rodrigo and Carina Damm
Sage and Colbey Northcutt
Cris and Rafael Justino
Norifumi (Kid) and Miyuu Yamamoto
Freddy and Veta Arteaga
Sisters in MMA:
Valentina and Antonina Shevchenko
BROTHERS WHO HAVE FOUGHT EACH OTHER
Caros Fodor vs Ben Fodor
Shunichi Shimizu vs Toshihiro Shimizu