If you don't obtain the ribbon in the fairy land, Syanna will be killed by Dettlaff. The second ending - death of Syanna - can be unlocked only if you decide to find her during The Night of the Long Fangs quest.
In respect to this, does romancing Syanna affect anything?
In The Witcher 3: Blood and Wine, you can romance Syanna who is a sorceress. However, do note that similar to Shani in The Witcher 3: Hearts of Stone, the entire fling with Syanna will not affect your relationship with Yennefer or Triss Merigold in any way.
Furthermore, should you tell the duchess about Syanna? When the duchess asks if you're happy with your reward, tell her about Syanna's plans (Got bad news – Syanna was out to kill you). When the duchess asks your opinion, tell her Syanna's clearly guilty. When Syanna lays out her defence and starts a counterattack, tell her You were children then, both.
In this way, does Syanna deserve to die?
Syanna doesn't deserve to die because of her travesties. She deserves to be punished under the rule of law, but not die. Dettlaff and his tendency to go off on everyone is not something you can excuse.
Where is Syanna after blood and wine?
Since she can be killed, I think Syanna just despawns after the main storyline is over. Lore-wise, I imagine she is in prison awaiting trial, then is either under house arrest in the duchal palace or is "imprisoned" on an estate outside the city. No way a duchess' sister would rot in a prison made for commoners.
