I used to wipe my feet on my sisters pillow
Why would you get someone else's things dirty?
She always used to moan and tell me off when I got my feet on her.
It is wrong to dirty something that does not belong to you. You have no right to annoy and demean someone with your feet.
I saw her doing bigger and more annoying things so it was just fun.
It still doesn't make it right to do. It is wrong to wipe your feet and dirty someone else's pillow. She might get revenge on you next if you keep doing it.