Author's Note: this isn't really supercorp but I got this idea from the latest episode and ran with it.
"I don't need a babysitter!"
"She doesn't need a babysitter!"
The Danvers sisters broke into complaint instantly after Eliza told Alex she couldn't go to that party after school that she'd been looking forwards to for weeks.
"Are you both forgetting what happened last time Kara was left alone?" Eliza asked, and Alex rolled her eyes and slumped dramatically in her chair. Kara turned a deep scarlet.
...
"Sorry!" Kara squeaked as soon as Eliza walked in through the front door. The woman stopped, mouth wide and eyes like teacups as she gazed about the room.
The kitchen was entirely trashed. The toaster was on the floor in pieces, one of the cupboards had the telltale signs of heat-vision-damage, and there was god knows what splattered everywhere, its source seemed to be the broken blender. Kara stood right in the middle of it all.
"What happened?" Eliza stuttered, slowly inching into the room.
"Um-" Kara began, but she was interrupted.
"Alex! Where's Alex? She was supposed to be looking after you!" Eliza began looking around as though Alex were hiding somewhere nearby.
"She, uh, I-"
Before Kara had time to come up with a viable excuse, Eliza was calling Alex's phone and already scolding her almost silently under her breath.
...
"Ok that's not fair. I just got frustrated and tapped the toaster in the side and suddenly it was flying off of the counter!" Kara defended herself, and Eliza hummed in disbelief, "And I tried to make a smoothie but apparently your blender had a bomb in it because it exploded everywhere and I got scared and accidentally heat-visioned your cupboard. None of that would happen again!"
"You're staying." Eliza instructed Alex, ignoring Kara's argument. The step-sisters groaned and flopped over the table in sync.