There have been many sibling tandems to compete together in the UFC.
Brother Teams 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) Cole and Micah Miller (Micah was on TUF 14)
20) Hector and Elias Urbina (Elias was on TUF 23 and CS 8), with Gilbert out there too
21) David and Daniel Teymur
22) Dominick and Alex Reyes
23) Ashkan and Suman Mokhtarian (Suman is on TUF 27)
24) Erick and Gabriel Silva
Sister Teams in the UFC
1) Valentina and Antonina Shevchenko
Brother/Sister in the UFC
TBA
Brothers by Marriage
Tai Tuivasa and Tyson Pedro
Chris Weidman and Stephen Thompson
Rolles and Roger Gracie - are they brothers? How are they related?
Brother teams in MMA but not the UFC
Yasuhiro and Toshimichi Akagi
Karl (Bellator) and Bertrand Amoussou
Kai and Mikuru Asakura (Both Rizin)
Chase (WEC, Bellator) and Carson Beebe (WSOF)
Gilbert (UFC) and Herbert Burns (One)
Alex (UFC) and Jose Caceres
Leon (UFC) and Fabian Edwards (Bellator)
Fedor (Pride, SF, Bellator) and Aleksander Emelianenko
Yasubey (FNG, ACB, KSW) and Felipe Enomoto (One)
Josh (UFC) and BJ Ferguson (Bellator)
Caros (UFC) and Ben Fodor (Phoenix Jones) are adopted brothers
Matt and Mal Foki (Both Pride)
Patricky and Patricio Friere (Both Bellator)
Pat (UFC) and Ryan Healy (Bellator)
Jeremy Horn and Matt Anderson are half-brothers
Enson (Pride) and Egan Inoue
Erik (UFC) and Keoni Koch
Naoyuki (UFC) and Hiroki Kotani (Shooto)
Ilir (UFC) and Arben Latifi
Muhammed (Bellator/SF) and Abdullah Lawal
Andrew (One) and Andrew Leone (Bellator, ACB)
Chuck (UFC) and Sean Liddell
Douglas (Bellator) and Dhiego Lima (UFC)
Lyoto (UFC/Bellator) and Chinzo Machida (Bellator)
Michael (UFC) and Brad McDonald
Gegard (UFC/Bellator/SF) and Gewik Mousasi
Takaharu and Takehiro Murahama
Anthony (UFC) and Chidi Njokuani (Bellator)
Alexandre Franca and Leonardo Nogueira
Khabib (UFC) and Abubakar Nurmagomedov (cousins, and adopted brother)
Alistair (UFC) and Valentijn Overeem (Pride)
Rousimar (UFC) and Rick Palhares
BJ (UFC) and Reagan Penn
Khalil Rountree (UFC) and Donavon Frelow (DWTNCS) are step-brothers
Mauricio (UFC/Pride) and Murilo Rua (Pride)
Chris and Andreas Spang (Bellator)
Marcus and Johan Vanttinen
Cousins in MMA:
Oliksiy Oleynik (UFC) 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 (UFC) and Carina Damm (All the Mans)
Sage (UFC) and Colbey Northcutt (LFA)
Cris (UFC) and Rafael Justino
Angela (ONE) and Christian Lee (ONE)
Freddy and Veta Arteaga (Bellator)
Naoki (UFC) and Mizuki Inoue (Invicta)
Gina (UFC) and Dave Mazany
Cindy (UFC) and Ben Dandois
Sisters in MMA:
Jocelyn Jones-Lybarger (UFC) and Jillian Lybarger
Family teams in MMA:
Brother Norifumi "Kid" (UFC), Sister Miyuu and son Arsen Yamamoto (Both Rizin)
Ray Cooper Jr. and Ray Cooper III (PFL)
Antonio (UFC) and A.J. McKee (Bellator)
Randy (UFC) and Ryan Couture (UFC/Bellator)
Jordan (UFC) and Lee Mein
Father Telman Piraev, and sons Marif and Akhmed Piraev
Ken (UFC) and Ryan Shamrock
BROTHERS WHO HAVE FOUGHT EACH OTHER
Jeremiah Castillo vs. Angel Castillo
Caros Fodor vs Ben Fodor
Shunichi Shimizu vs Toshihiro Shimizu
Thomas Treadwell vs. Mike Treadwell