Spoiler Free except what is implied by the achievement description.
The
Yet Here We Are Achievement is unlocked automatically through normal story progression. It is triggered by the
Marcano's Fate mission in the Hotel Paradiso. The achievement will not unlock immediately upon exacting revenge for Sammy and Ellis. First, Lincoln must leave the Casino. After leaving the Casino, some cut scenes play out. At their conclusion, the achievement unlocks along with a new story mission--
“Before They Bury You”.
Below is a video to indicate where the achievement pops. The content has been edited to avoid spoilers: