The "Jo" is the "Yes" in your sentence. However, it is a little different from the usual "yes". Let's say you only answer "yes" to the question. Then, would the one asking know if you have sisters or not? It's quite confusing. The "Jo" is an "Ja" used for this particular situation (so you don't have to say the "I have" part).
In Farsi we have the word "chera" in the contrary forms and I haven't seen such a thing in english so u may not feel how this happens. In informal english if u ask a question like this: "You are not ok?" the Yes or No reply can both mean that u r not ok. So u should explain more: "No/Yea I'm ok" Norwegians use Jo here. Jo = in the contrary I am good.