Say I have a Perplexing Chimera.
Text: Whenever an opponent casts a spell, you may exchange control of Perplexing Chimera and that spell. If you do, you may choose new targets for the spell.
Then I bestow Hypnotic Siren on my own Chimera (I still control it).
Now when I use the Chimera to steal my opponent's spells, I think the siren will keep it under my control, because I'll still control Siren as an aura. I can gain control of everything my opponent casts.
Does this work??