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I am sorry to hear about the problems you are experiencing with your father in law. Bipolar disorder can be a very difficult condition. You may find the following link helpful in understanding this condition: http://www.nimh.nih.gov/health/publications/bipolar-disorder/complete-index.shtml
The advice that you have been given is unfortunately correct. Unless your father in law is an imminent danger to himself or someone else, there is nothing you can do to force him to seek treatment. If he would make threats to harm himself or others, or if his situation deteriorates to the point where his health is imminently endangered by his behaviors, then they could contact authorities to have him hospitalized. However, it is important that his wife and daughter set the limits that they need to in order to safeguard their emotional health. They have the right to limit contact, or set limits on what would be required in order for them to visit, etc. The only way that he might choose to get help or to change behaviors is if no one enables him by giving in and participating in these behaviors.
I hope this is helpful. Please let me know if I can clarify further.