Sweet Iona / Sweet Dairren

Background
You can bed either Iona or Dairren, but not both. Making plans with one will cause the other to refuse but after its all over, both quests, "Sweet Iona" and "Sweet Dairren" will be logged in your completed quest section of your journal.

Walkthrough
Follow Iona and Dairren to the study after talking to your mother. Talk to either Iona or Dairren to eventually bed them. You have to be blunt in your choices.

Result
Either Iona joins you in bed that night, or Dairren does.