I'm frustrated with my sister over her poor life choices
What are the poor life choices that your sister made?
She has gained weight, she always says bad things about me and she is the reason I'm missing my high school friends.
You and your sister should support your family members and be more understanding and less critical. Your sister probably does not agree with all of your life choices either. Family members should be there for each other.
Don't tell me what to do.
Okay, I can listen if you just want to let your feelings out.  However, I would suggest that you forgive your sister for some things, and try to talk to her about the others.  Most of her decisions are her own business.