I am going to ignore an agreement I have with my sister.
What is the agreement with your sister?
I won't say things behind her back and tell everyone on a public forum on a website.
It is important to keep your word when making a deal. If she found out she could be really hurt or try to retaliate.
I'm not agreeing with you, like you won't agree with me.
It is still rude to break a deal with someone. You should honor your sister and respect her wishes, because she might have dirt on you.