The Everything Health Guide to Adult Bipolar Disorder: A Reassuring Guide for Patients and Families By Dean A. Haycock:

If someone spends a great deal of their life caring for or worrying about someone with this illness, he might find his own accomplishments and sense of satisfaction limited. The goals and plans of the caregiver may fade as he instead tries to save or protect the person he looks after. A person with bipolar disorder can take over a caregiver's life if the caregiver does not protect himself.

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An even worse scenario for some caregivers is not just giving up their own goals and plans but, if  it comes to divorce, having their children taken away from them by the bipolar parent despite the courts giving the caregiver custody, as happened in the case of my daughter.

The bipolar person is often able to persuade some he comes into contact with  that he/she is perfectly normal and balanced and that the other parent is the only one who is ill. They may even think that by remarrying they are replacing the other parent with a step parent for the children. Thus the children are no longer in the care of the caregiver but in the care of a bipolar mother or father and a step-parent.

People with bipolar do not always recognise that their illness has driven the caregiver into depression perhaps because they are self centred and can only see things from their own viewpoint. While depression does not lessen the love the caregiver has for their children it does give the children a picture of one parent's weakness and loss of stength and because the bipolar parent comes over as being the stronger willed  their children often adapt themselves to what they believe will keep that parent on their side. Being so young and inexperienced they are unable to deal with the withdrawal of love. They only know they need fear no loss of love from the depressed caregiver but fear the loss of their bipolar parent's love.