I would make it all about me so my mom didn't think the focus was her or her mental health. I would say something like, my doctor is concerned about X, and wanted me to ask my closest relatives if they had ever experienced such and such. This might help give him some clues about my treatment. I'd even have a list of objective, clinical questions to ask her, so it didn't seem personal. And if she did answer yes to any questions, I wouldn't delve into it with her but just note it on my list. The family history can be really helpful in psychiatry so this is well worth doing, I think. Good luck!

You might even wait until you see the pdoc and ask his advice on how to proceed and what he would find most useful to know. That way if your mom asks about why you want to know you can say your new doctor requested you get the information to help him. I think I would probably wait until I saw the doctor so I wouldn't have to go back to mom for more questioning in case I hadn't thought to ask something the new doc wants to know.