Santana is the bad bitch and Blaine is the golden cupcake. That is how they work- she snarks and protects him in her roundabout way, and he keeps her happy and not killing anyone.
That's how it is, at least until Santana's little brother, her baby brother who she's always protected with a fierceness unmatched in anything else she does, is attacked at a school dance. That's when Santana realizes that she can't do anything by herself and that she might need to let him go somewhere else, somewhere where she may not be able to check on him daily but where he'll be far safer.
That's all she's ever wanted, anyway- she wants Cooper and Blaine safe. No matter they're her step-brothers; family is family and Lopezes protect their own.