We know canonically that the Alliance won Darkshore but as always the commander (Sira) kneels before the victors not fall on their bellies and die. So I could see Maiev killing Sira but I could also see her imprisoning her to help save her maybe.
I do think it was more likely that she "freed" Sira by cutting her down. Meaning Delaryn and the remaining Dark Rangers are all that are left of Sylvanas' raising in Darkshore probably.
