I don't think that's the same thing. The primals seem to do it because they're gods, however, Siren seems to do it because of magic. I don't know... Also, there are multiple Sirens, as in, a full race of them.

Also, I was thinking that in the fight, if she happens to kill a party member, she could use them as a thrall maybe? She does control the dead, so why not you?