It is increased because of your sister's diagnosis. You share 50% of your DNA with your sister, which is as much as you share with each of your parents. I'm not sure what your chances of getting bipolar are, but they're probably not less because it's your sister and not one of your parents (should be about the same). There is also a correlation between family history of unipolar depression (and, less so, MI in general) and bipolar, so the family history of depression would increase the statistical chance even more. I doubt you will find much good information beyond the statistical chance of getting bipolar when one of your siblings has bipolar though. Your chances would be higher than that, assuming only family history is considered. You shouldn't worry too much about this though. It probably won't happen.

Here's one relevant article:
Family History Best Indicator That It’s Bipolar Disorder, Not Depression | Psych Central News