I'm going to be cold to my sister
Why would you be cold to your sister?
When i was young she made fun of me in front of my friends.
I see your point, but you shouldn't be mean to your sister. You should treat family with respect.
No. What is she going to do to me?
It is not super reasonable to treat your sibling badly about something that happened when you were a child. You are really holding onto a grudge here. If it still truly upsets you, then maybe talking to your sister about it would be helpful.