I'm planning to purposely put my red sweater in the washer when my sister is washing her new white pants.
Why would you want to do that to her new pants?
What's the big deal?
The "big deal" is your sweater will likely give off dye and color her new white pants red or pink. She will likely not be very happy with that.
I'm better than her, I will fix the problem if that happens.
She's your sister, it's not ok to ruin her pants purposely. Put yourself in her shoes and think how you would feel. Leave her alone and focus on doing something productive.