I'm a bit confused about the relationship between Thomyris and Darius

I might have missed some lore items but is Thomyris supposed to be Darius' wife, with Vahram being her step-son? So if she wanted her own kid on the throne, why not just have a few children with Darius first, then murder Darius and Vahram? Surely the crown wouldn't pass to her but to another male relative.