Of course sisters can both have bipolar disorder. If the gene is strong in a family, there is a likelihood that many in the family can develop it. Unfortunately, that has been the case in my family. My paternal grandmother had a confirmed BP, my dad, I do, my sister, and my youngest nephew. I also have a 1st cousin with a BP dx. The severity and general characteristics/course/nature of our BP cases have varied a bit among us.

Others in my family have also had signs of possible bipolar disorder or at least depression and/or anxiety.