Well, the science behind takes those possibilities into consideration.
The studies include children adopted into bipolar households, and they do not develop bipolar disorder as often as biological children; and also children of parents with bipolar disorder adopted by non-bipolar and relatively healthy families that develop bipolar disorder nonetheless. These studies also include twins with bipolar disorder predisposition who, even when separated and given different environments, both develop bipolar disorder.
Stress plays a part. For many with predisposition, stress gives the push. This is known as the diathesis-stress model. And, yes, some people develop bipolar disorder without known bipolar relatives.
Yet:
Bipolar disorder is frequently inherited, with genetic factors accounting for approximately 80% of the cause of the condition. Bipolar disorder is the most likely psychiatric disorder to be passed down from family. If one parent has bipolar disorder, there's a 10% chance that their child will develop the illness. If both parents have bipolar disorder, the likelihood of their child developing bipolar disorder rises to 40%.
From: https://www.blackdoginstitute.org.au/resources-support/bipolar-disorder/causes/
*Social anxiety intensifies*