Comparing this situation to the Ciri one is a poor comparison. Kaer Morhen is a defensible position that can be prepared ahead of time to draw in an enemy, not walking into the power of an unstable monster. Further the fact that Syanna was in danger from Dettlaff was pretty clear from his actions and demeanor and sure enough if she is brought to him she dies unless teleported away.
Besides that there are other reasons beyond not being willing to send her beloved long-lost sister to her death. As every ruler knows you cannot simply give in to threats. If the Duchess of Toussaint is intimidated by the threat of violence this time then that's going to encourage others to follow suit. After all if she gave up her sister what else might she give up? You may have heard the cliché "No negotiations with terrorists," this is why.
When someone threatens violence and carries through on that threat they and only they are responsible. Blaming Anarietta for Dettlaff's slaughter is like blaming the victim in a mugging for getting shot because they refused to hand over their wallet.