Lots of things could have caused the bipolar.
The genes have to be present in the person for bipolar to manifest.
At some point the genes are triggered or turned on -- for example hormones.
Twenty-five is the average age of onset
Other things that can trigger bipolar ---
corticosteroids, anti-depressants, stressful events
It is possible that a virus can trigger bipolar.
Do a search without chicken pox -- just do virus and bipolar.
With good treatment from a fantastic psychiatrist at a major medical teaching hospital, your niece can get well.