Just and only can mean the same thing, but they do have differnt meanings too.

Meaning 1 (they mean the same thing) I just have one sister = I only have one sister

Meaning 2 (when we are talking about an action that has been performed very recently) I = just got here, I just ate, i just had a baby.

Here is a good explanation

http://www.espressoenglish.net/difference-between-just-and-only/

just is I just arrived acabo de llegar in that momento en  este momento como acabar de just finished the work  acabo de terminar el trabajo.  I just spoke to my father.  acabo de hablar con mi padre.

I only want to do this job.   I only want to study and nothing else. nada más

me quiero solo trabajar en este trabajo.  only como se hace este trabajo este estudio y no más que la otra.