Westerado: Double Barreled

You'll gain this achievement once you unlock your Brother, your Sister and the Indian.

To unlock your Brother, just complete the complete the game with the Hero. You should have a notification he's been unlocked after your final confrontation with the Murderer.

To unlock your Sister you'll have to complete the questlines that bring a train to Clintsville. Begin by talking to the Ticket Seller at Clintsville Station. He'll eventually admit that he's been hiding the Train Master's mail because he enjoys his lazy job. Tell the Train Master this and (once she's yelled at the Ticket Seller) she'll ask you to clear the tunnel of bandits.

There are actually a number of ways to do this, but if you want the most direct method just follow the tracks west until you come to the area with the cave in and shoot all the Beefhead Bulls nearby. Then head up the ladder to the north and clear out the rest of the gang.

Note that this will prevent you from gaining any of the other achievements concerning the Beefhead Bulls this playthrough. If you have $300 on hand you can also pay the Bulls off by talking to them at the cave in.

Once the area is clear, return to the Train Master and she'll ask you to find three people to assist in clearing the rubble. While the mission suggests asking Ranchers and Miners, there are actually several random NPCs in both Clintsville and Santa Anna who'll help with the task. Just ask around, and once you've found three helpers return to the site of the cave in. The Train Master will confirm the quest is complete, and ask you to meet back at the Clintsville Station. Once you head back there she'll give you a letter from your Sister saying she'll be on the next train into town.

Go and clear some other missions and check back at the station regularly to pass time. Once the train arrives, your Sister will be standing next to the station door. Talk to her to unlock her as a playable character.

Finally, to unlock the Indian, you have to help the Indians take over Clintsville. I'd do this in a new save slot, as it will lock a great number of quests out of the playthrough involved. Start by heading to the Indian Outpost (east, then north, then east from Clintsville) and talking to the Chief. When he asks why you're out there say no reason, then ask if he needs help. He'll tell you to free the buffalo taken by ranchers to help his people.

The are four Ranchers in the game, and assuming you're using a new save slot, the simplest way to free three of their herds is to go to their ranches and kill them. I'd avoid doing the Rancher next to the bank's herd, because it'll make the city of Clintsville hostile (before you need it to be). So head to Rancher Bob's (NW of Clintsville), Rancher Cobb's and Rancher Richard's (both SW of Clintsville) farms, shoot the lock off their gate and herd the buffalo out of their pen. You'll know you've herded them out when the mission status updates.

Once you've freed three herds, return to the Outpost and talk to the Chief. Ask if you can help further and he'll tell you his people are about to attack the fort. He'll give you a drum and tell you to bring it to his brother in the Indian Camp. Enter the mine just above the Chief and go East, North, East, East, South, East, South, South to reach the Indian Camp. Give the drum to the Chief's brother (in front of the teepee) to show you have the Chief's trust, then follow them south to help them fight the soldiers.

This battle is a pain in the butt, but at all times while you fight avoid shooting any Indians. I glitched the battle a few times by accidentally killing one of my Indian friends, which made both sides hostile to me. It doesn't seem to matter how many Indians die during the battle, as long as you're not the one to kill them. You'll have to fight four or five waves of soldiers before being declared victorious, so don't leave the screen.

Once you win the battle (the journal will update saying you've failed Soldiers vs Indians, but completed Indians vs Soldiers) head south to the Fort and talk to the Chief once again. You'll unlock the Tomahawk for your trouble. Ask if you can help further and he'll tell you the Indians want to take Clintsville. Agree to help once again and this will take you directly to Clintsville to fight with the population. Once again try to avoid killing any Indians in the ensuing firefight. Once the battle is complete the Chief will aware you a feather hat, and this will unlock the Indian for use in the main game. If the Indian is your last character to unlock, the achievement will pop here.

If anyone has a potential shortcut to any of the methods above, let me know and I'll update the solution!