When The Cows Come home

Detailed Walkthrough
Find Clayton standing around at Oberland Station. He will ask you to get Sarah to have sex with a nearby Brahmin.

You can convince her through speech a few different ways. It doesn't matter which way. You may also be able to arouse the Brahmin.

Next, find Clayton in Diamond City, and tell him the good news.

You will get a quest update once Sarah's pregnancy has progressed. When going to talk to her, Carmen will arrive at Oberland Station, sometimes pursued by a few hostile Gunners. Kill any Gunners pursuing Carmen, and then watch the conversation.

Speak with Sarah, and she will confess that she is attracted to Carmen.

Speak to Carmen, and she will express the same about Sarah. If you are quick enough, you can catch Carmen before she leaves Oberland Station to talk to her. If not, she can be found at the old produce market stand down the road that contains a cooking station and a chemistry station in the back. The market stand is near the ruined town beside Chestnut Hilcock Reservoir, which is outside of Vault 81. The ruined town frequently spawns random enemy encounters, so it may be best to deal with the enemies before attempting to speak to Carmen and escort her back to Oberland Station, in case a group of enemies migrates toward the market stand after surviving a fight with another group of enemies.

Carmen asks you to kill the Gunners that are after her that aren't already dead. It is possible to bypass this with a speech check. After either killing the Gunners or just convincing Carmen to stay, the Sole Survivor must wait a few days for Carmen to settle in.

After Carmen has settled in, a quest update will be received. After traveling back to Oberland Station, she can be spoken to. Carmen will want to help make the settlement a safe and prosperous place, and she will ask the Sole Survivor to do some things to make that happen.

Securing The Settlement
The first thing Carmen will ask of the Sole Survivor is to find someone willing and able to protect Oberland Station in case Clayton tries to wrestle them out. Depending on the Sole Survivor's progress in the Commonwealth, additional options become available:

An option that is always available is to find Ivan. Ivan, an ex-Gunner that left with Carmen, can be found in Goodneighbor inside of The Third Rail. Through conversation, he can be convinced to move to Oberland Station and to help guard it.

Aside from Ivan, the Sole Survivor can enlist the help of the Hornets, given the quest "The Hornets' Nest" is complete. To enlist the Hornets, the Sole Survivor will have to talk to Paladin Bourne.

The Sole Survivor may also enlist the Blackwood Rangers, if they have completed the quest "Finder's Keepers". To do this, they will have to find Camilla, formerly known as Aries, and ask her to move in at Oberland Station.

Lastly, the Sole Survivor can order the Minutemen to send a small garisson. To do this, they will need to be made General by Preston Garvey, and then talk to Preston.

Setting Up Trade
Once the town is secure, the Sole Survivor must speak to Kessler. Kessler can be found at Bunker Hill. Through conversation, she can be convinced to start sending caravans to Oberland Station.

Returning to Oberland Station and speaking to Carmen after she and Clayton have an argument will complete the quest.