Brothers and Sisters, One and All is an optional story mission in Red Dead Redemption 2 given by Brother Dorkins, as part of Chapter 4.
It's the 48° mission in chronological order of the RDR2 story mode, played with protagonist Arthur Morgan.
While optional for story progression, this is an Optional Honor Mission required for the "Lending a Hand" Trophy/Achievement. It's a time-sensitive and missable mission, so it can only be played at this specific point in the story.
Brothers and Sisters, One and All - Mission Details:
A street kid steals Sister Calderon's crucifix. Arthur chases the kid through the streets of Saint Denis to get it back.
Mission Objectives
Gold Medal Objectives (100%)
The following are the Mission Objectives required to obtain the Gold Medal in the mission "Brothers and Sisters, One and All":
- Find the kid within 19 seconds
Note: You must complete all Gold Medal requirements in one single playthrough of the mission.
Brothers and Sisters, One and All: Mission Info
- Mission Number 48
- Story Chapter Chapter 4
- Mission Giver
Brother Dorkins - Mission Type Missable Mission
- Protagonist (Playable)
Arthur Morgan - Location Saint Denis (Bayou Nwa, Lemoyne)
- Rewards Honor
- Featuring
Edith Downes 
Sister Calderón - Notes Optional Honor Mission, required for the "Lending a Hand" Trophy/Achievement.