Talking with Beckett at one's C.A.M.P. He suggests going to Watoga Underground, where the player character can speak with Beckett and Ronny. One will be given the key for future access. Enter the underground with Beckett and fight through the enemies. Eventually, one will reach a big red button to open the garage door.
Press the button and kill all remaining Blood Eagles and their robots. Several raiders from Edwin's gang will help in the fighting. Afterwards, proceed to the Claw in a nearby room, who turns out to be Frankie Beckett. Resolve the situation with Beckett, either by killing Frankie or convincing Beckett to spare him. Afterwards, return to the C.A.M.P. and talk with Beckett to complete the quest.
There are several security terminals with an option to open the garage door, but these options do not progress the quest.
Behind the scenes
"Thicker than water" is a shortened version of the phrase "Blood is thicker than water," a medieval proverb meaning that blood relationships will always be stronger than bonds of friendship or love. The quest title refers to the choice that Beckett must make, to save his brother or kill his brother.
[verification overdue]Beckett will fail to recognize that Frankie was spared after returning to one's C.A.M.P.