Dependency

Dependency is a side quest in Fallout 4.

Detailed walkthrough
Upon entering the reactor room of Vault 81, after completing Hole in the Wall, Tina De Luca can be found working. Talking to her, she thanks the Sole Survivor for helping Austin and proceeds to ask for help getting her brother, Bobby De Luca, to kick his Jet addiction. Since the incident involving Austin, his addiction has gotten worse and Rachel from the infirmary has warned that if he doesn't get clean soon he could die. Rachel has a treatment for Bobby, and Tina asks the player to convince him to go see Rachel.

Bobby is found in his room inhaling Jet. He questions why he's being interrupted and there is a choice to be either compassionate and convince him to go see Rachel, or aggressive which can cause Bobby to go hostile and attack. If Bobby goes hostile the Survivor will have to kill him. Returning to Tina with high enough charisma and you will be given the option to pass a hard speech check to convince her to join a settlement. Alternatively convince him to see Rachel, and he will walk to the clinic where Rachel can be informed of his choice. She'll compliment the player on being a good friend helping Bobby overcome addiction (but do not go see Rachel with Bobby if you want Tina to join a settlement - see below). Once Bobby arrives she explains the process to him and administers Addictol.

Return to Tina to complete the quest. Afterwards, there is an option to recruit Tina as a resident in a settlement as she states she is sick of living in the Vault and wishes to see what the Commonwealth has to offer. If you go to see Rachel before Tina, then the quest will be complete and you will not have the option of convincing Tina to join a settlement.

Bugs

 * If Bobby is alive, even if you successfully invite Tina to your settlement and then finish the quest by talking to Rachel she will remain in the Vault 81. If you talk to her again she will mention "seeing the Commonwealth" and "finally getting out" but she won't join the player anyway. This can be bypassed by insulting Bobby until he attacks the player. Tina will join the settlement after passing a speech check. In short, the only way to successfully recruit Tina is to kill Bobby.
 * A workaround without killing Bobby via console commands: Go to one of settlements, open console and type, and enter workshop mode and move Tina to preferred settlement.
 * Possibly related to the above bug, successfully completing Tina's speech check to invite her to a settlement can lead to the quest becoming impossible to complete, with the objective to speak with her not not marking as complete and it being impossible to initiate conversion with Tina again. Going to talk to Rachel (who has no marker) seems to allow this to be bypassed and the quest to be completed.
 * After talking to Tina, you can go to Rachel as well as Bobby. Going to Rachel first and asking about the Addictol can lead to her asking you to bring Bobby to her. However, going to Bobby reveals him in his room, inhaling Jet and refusing to respond to you at all.
 * Rather rarely, even after saving Austin, Tina won't tell the player about her Brother's addiction. The quest will never start.

Зависимость (квест)