Mission 1: Oh Brother
Vance is found in Nassau. Head to the location and find Upton's men. You have to eavesdrop and tail them so hire the nearby dancers and follow the two. They will lead you to a restricted location. Head to the rooftop and use your eagle vision to identify your target. You don't need to kill any guards here; just move to the nearest rooftop to get Upton within your jumping range and attempt to assassinate him. After the cutscene, the mission is complete.
Mission 2: The Other Brother
Meet up with Upton and follow him until he meets up with Vance. Hide in the nearby foliage and tail them. Climb the rooftops to get a better view. Not long after, a thief will appear. You have to eliminate the fella stealthily (make sure to hide his body to prevent any passing guards becoming suspicious) and quickly returning to observe the brothers. There's a tool cabinet in the market area; wait for the thief to move closer then make your move. Carry the body to the cabinet to hide it.
Next, head to the gallows in advance and wait for another thief to appear. Make sure to eliminate him when he's right beside the gallows. The two guards behind you are not looking and if the thief makes it to the front, there are two more guards which will make it harder for you to move. You can hide the body inside the tool cabinet at the back of the gallows.
Climb the rooftops again and observe them. The third thief will appear in the alley, conveniently near another tool cabinet. Execute him from above then dump the body in the cabinet.
This time, you have to tail Vance until he meets up with a woman pirate. Hire the nearby dancers and get near the two to eavesdrop on their conversation. Your objective will shortly be updated and you have to find Upton's assailant before the time runs out. Head to the marked location and use your eagle vision to quickly identify the assailant. Kill him before he makes his move to complete this mission.
Mission 3: Upton's Sorrow
Head to the tavern to start this mission. After the cutscene, you'll be in a brawl. Defend Upton and defeat all the thugs. Once done, you'll have to escort him to safety. Keep following him and he'll eventually decide to take a piss in the plain view of incoming guards. This will instigate to a fight; if you're unlucky, the five-man patrol will take the road and join the fray. Just use your smoke bombs to quickly take the guards out. The mission will be complete shortly after defeating all of them.
Mission 4: Queen of Pirates, King of Fools
Meet up with the now-sober Upton. After talking to him, head to the swamp and take out the guards as you try locating Vance. Use your eagle vision as well to mark any roaming crocodiles in the water. You'll find Vance in a camp west of the swamp. There's just a couple of guards nearby; you don't need to be stealthy if you want. You can even let them kill each other or give the roaming crocodile(s) nearby a free meal.
After Vance's death, you'll now have to chase Jing Lang and kill her. She's not a fast runner; you can even use your darts to stop her tracks and make it easier for you to land your finishing blow. The mission is complete after her death scene. You'll have another Templar Key in possession.
video: TH_Nass4C.MP4