Quite honestly if you have anxiety, even though you don't need medication, you may want to try therapy. You can space it out to every 2-3 weeks but you'll have a mental health professional helping you w. Your anxiety. They can watch out for bipolar symptoms so that it'll be caught early which lessens the severity.

As for running in families, 2 out of 3 of my siblings are dx with bipolar or scizoaffective bipolar type. My younger sibling suffers from mood swings and psychosis but refuses to get diagnosed because "they'll never let me out if they know what goes on in my head ". My extended family is plaged by addiction or untreated but dx'd mental illness, this includes my parents. So the chances of myself or my son escaping mental illness were slim. However it seems like only your sister has bipolar but you have anxiety. So as long as you're in treatment for your anxiety I would not add this to your concerns.