The Starkhaven Longbow
The quest giver: Automatically after finding the Starkhaven Longbow [Hightown - Harimann Estate]
Step 1: Find the bow
A chance of finding the bow appears during the completion of the companion quest Repentance. You have to get to the cellar in the [Hightown - Harimann Estate] location. After dealing with the monsters #1 examine a body of Flint Company Mercenary #2 (M97, 7). You'll find the Starkhaven Longbow among other things. Further actions will be possible only after completing the quest Repentance that is after leaving the [Underground passage] location.
Step 2: Give Sebastian the bow
After completing the companion quest Repentance stay in the [Hightown - Chantry] location and click on Sebastian (M7, 5) #1. You can ask prince about the bow and his grandfather #2. Depending on the main dialog option you've chosen you'll receive 10 friendship points (upper right), 5 friendship points (middle right) or 10 rivalry points (bottom right) in relation with Sebastian. Note also that the Starkhaven Longbow is automatically exchanged for a weapon he was using till now.
Questioning Beliefs (Sebastian)
The quest giver: Sebastian [Hightown - The Chantry]
Step 1: Talk to Sebastian
This quest will automatically appear in your journal after you accumulate enough friendship or rivalry points in relation with Sebastian (Exiled Prince DLC required). The easiest way to modify your relationship is to take Sebastian within your party when completing quests. If you want to obtain friendship points then make decisions after his heart and if you want to obtain rivalry points then make decisions which he doesn't like or even make him angry. After this quest appears in your journal go to the [Hightown - The Chantry] location during the day. Click here on Sebastian #1 so you can talk to him. The main subject of this conversation will be probably prince's hesitations about what he should do with himself. The game will also allow you to continue romancing with Sebastian #2.