I'm planning to hide my sister's bra.
Why would you hide your sister's bra?
No big deal. I just want to see if she would notice it or not
I feel like this is something that she will notice. Even though it is just a joke, this may not be a great idea.
It's not that serious.
Even though it is nothing huge, this may be something that makes your sister upset. Try to keep her feelings in mind here.