Actually, I don't WANT to unvampire Sera.
I think it would be too difficult with Lilumae in Winters life now. I think that Lilumae would feel unwanted/uneeded while Sera is around. And Sera would constantly view Lilumae as her replacement in her absence, or her competition.
As interesting a dynamic as that would be, I much prefer the route as it stands. I think it would be better if we are able to save Simus daughter from becoming a vampire *or cure her if she already has become one* but the cost would be Seras life, for some reason. Maybe the two girls are interconnected, like, Sera is the one who turned Simus daughter into a vampire, and by "curing" his daughter, you end up destroying Sera via the connection they had.
This allows Winter to finally put her sister to rest, for good, and to also have Lilumae by her side without the guilt of inadvertently forcing a competition between Sera and Lilumae.
That's my current idea, anyhow.