She is just giving Iz more of a reason to want power at this point. If she made an ally out of Ruby, and seeing how taken she was with the princess, she can use Ruby and her kindness to her advantage, giving her the best odds that Izek would not want to harm them in the future. Even if he were to want power, he wouldn't get it at the cost of hurting Ruby by taking away her allies.
By betraying Ruby, she's just made an enemy out of Omerta, putting her daughter more at risk from the very thing she's trying to prevent.
She is just giving Iz more of a reason to want power at this point. If she made an ally out of Ruby, and seeing how taken she was with the princess, she can use Ruby and her kindness to her advantage, giving her the best odds that Izek would not want to harm them in the future. Even if he were to want power, he wouldn't get it at the cost of hurting Ruby by taking away her allies.
By betraying Ruby, she's just made an enemy out of Omerta, putting her daughter more at risk from the very thing she's trying to prevent.
It's so frustrating aghh Σ(っ°Д °;)っ