What's your MC's Charm Factor?  That has a large impact.  If it's Terrible, you might not be able to convince her no matter your dialogue choices.  

During the first conversation, these dialogue choices will give "sympathy points":

• [if planning to enslave her] "I promise to treat you fairly":  +1
• "I'm sorry for your loss, but you have to think of yourself now":  +1
• "You've tried to do what you thought was right":  +3
• "If anything, you would still be able to help people instead of dying":  +3

If your total sympathy points plus your MC's Charm Factor (where Terrible=1, Poor=2, etc.) is at least 10, you have a chance.  

During the second conversation, if you succeeded the first day, there's a second sympathy counter:

• Aire is alive, you reunite them, you saved Aire during the Fighters Guild quest and treated her well:  +3
• Aire is alive, you reunite them, you didn't do the Fighters quest:  +2
• Aire is alive, you reunite them, you abused her:  -2
• You return her bracelet you retrieved from the rebel base:  +1
• "I promise to treat you with dignity":  +1
• If planning to marry her:
  • "I fell in love", Charm at least Good:  +2
  • "I can't stop imagining you naked", Sex Factor at least Great:  +2
  • "How could anyone pass on a girl like you?":  +1
  • then:  "You can take it at your own pace", Charm at least Average:  +2
  • "We can work on it...", Sex at least Good:  +2
  • "I understand...":  +1  
• If planning to enslave her:
  • "I'm sure your charisma...":  +1
  • then:  "Naturally, but....", Sex at least Good:  +2
  • "To be frank, ...", Charm at least Good:  +2

To succeed in convincing her to marry you, your sympathy + Sex factor has to be at least 11.  To succeed in convincing her to become your slave, your sympathy + Charm factor has to be at least 8.