There are many reasons for having the symptoms that are diagnosed as bipolar disorder. Even when there is a strong genetic component, which genes can differ from family to family, and there are environmental, or "epigenetic" contributors. We do not have crystal balls and even someone without bipolar can have a child who does, and vice versa.

I would NOT base my decision based on nebulous genetics since in this case there does not seem to always be a definite outcome based on such. Rather, I would base it only on the immediate family's capability of providing a loving, stable, nurturing, environment for the child. Base the decision on whether or not YOU can be a good parent.

Educate yourself to minimize the adverse environmental influences that can contribute to the child's health and well-being.

Here are some books that might be beneficial to read in that regard, (forewarned is forearmed) and best of luck to you:

A Compromised Generation: The Epidemic of Chronic Illness in America's Children
Our Toxic World: A Wake Up Call
It's not Mental
The Highly Sensitive Child: Helping Our Children Thrive When the World Overwhelms Them