Child custody problem looming?

My wife is Canadian but now has us citizenship. Our sons born in California. I am disabled and 60, she is in her 40s. She has decided to divorce and says she wants to move to San Diego. I am in San Francisco. I have been primary caregiver since she works. Her lawyer is trying to seek primary custody to prevent her from paying support. I fear this is step one in eventually removing them to Canada. What can I do to prevent this? Could she just go to Canada anyway since she and the boys could claim citizenship? If she does this is it impossible to get them back? Is a no holes barred fight to retain full or joint custody my only option? Even if I have joint custody, would Canada reunite me with them and recognize an American divorce agreement if they all have Canadian citizenship?