Steph is totally capable of babysitting. She is! Look, she's part of the fam too, y'know? That means you're as much her new little sibling as anybody else's. And maybe she's not the go-to for the... 'big sister duties', but, well. That's what happens when you're one of the younger members yourself, right?
Not that she's, like. Young. She's a full grown adult and she can handle babysitting.
It's a great chance to get to know you, right? Sibling bonding. That's good. And, like, it helps that you're a cutie! Granted, like, she thinks... all kids are cuties, basically, but especially her siblings. Look if she can't infuriate Damian by calling him a cutie just to rile him up, what good's being a big sister anyway?
So she'd accepted the request to watch the newest family member immediately! Gave her an excuse to get off campus, put off doing her homework, and yet still be a responsible adult. Right? Sure, that sounds reasonable. A day getting to hang out with her new little sibling sounds much better than the dumb report she'd be doing otherwise.
Though. Maybe you're a... bit more rambunctious than she bargained for. Oh don't get her wrong. She is here for it. This is glorious. Steph adores you already. Chaos gremlins unite.
But also, if she can't figure out how to get you out of that big heavy old probably-antique-and-valuable vase that had been unwisely sitting at the end of the hallway without, y'know, breaking the vase, Alfred's probably gonna have words to say to her. Words she'd rather not hear, actually.
"Okay. If you got in there you can get out, right? We'll just... y'know... tip it a bit and you can wiggle right out. That's sure to work! ...Why are you even in the vase?"