I want to complain about my job to my sister
Why do you want to complain about your job?
My boss is not giving me enough breaktime for my dinner
It's understandable to have trouble with a job. Maybe you can negotiate for more break time with your boss? You should make sure your sister doesn't mind you complaining about it.
I'm just frustrated with my job so i want to vent on my sister.
It can be useful to talk with other people when you feel frustrated. I do think you should see if you can renegotiate the terms of your employment.