In order to Romance Sybil you must be a male character, finish all her side quests, and have her in your party when you slay the Creator concubine in Act 3 Chapter 2.
You will also get the achievement Patron if you go for this.
*You can do this while on the demon path, I recommend it actually so you can also romance Edwen without having to play through an entirely different game.
There is no real secret to this, simply finish all her side quests without failing. During the Orphan quest you can select the wrong dialog which would in turn fail the quest. There are no wrong things to say to Sybil other than that quest. Besides conversing to get the quests there is no other necessary dialogue topics you need to discuss with her.
If there is a red X next to the side quest title that means you failed it and this will be unobtainable without either restarting the playthrough or reverting to a previous save where you can still finish the quest.
Sybil's Quests are - for more detailed quest info, see the guide for PATRON.
1. On Safari with Sybil - show Sybil local fauna
2. Orphan - Cheer Sybil Up ("I'm sorry about your dad.")
3. The Icy Library - collect the 3 books
4. Timid Request - talk to Mathas about his sex life
5. A Damsel in Distress - find and help Sybil
Now take Sybil with you during the Chapter 2 main quest - Witch hunt - Kill the Creator - this one is on the right side of the first map area.
***SAVE BEFORE DROPPING INTO THE BATTLE ZONE - so you can revert back here after getting this achievement so you can romance Edwen as well. You will have to fight the Creator again then pass on Sybil's advances in order to be able to romance Edwen. you cannot do both on the same game, this is why the save revert is necessary.
The Creator will call in the Stalker concubine if you haven't killed her already so be prepared.
Once the Creator concubine battle is over you will be immediately involved in a conversation with Sybil. When you get the option to choose a dialogue response choose, "Well, I like being with you, too, you know." and that's it.