As someone who works in the Behavioral Health field, I hope that your mom is getting all the best care possible.
I'm a Supervisor at my job, and I could NEVER imagine being as rude, ignorant, and insensitive as your bosses! Like many have said on here, they have NO RIGHT to demand to know the medical situation of your mother. Medical information privacy is covered by the HIPAA act.
http://www.hipaa.org I cannot believe they pressured you to tell them what was going on.
I understand that you feel uncomfortable going to HR about your boss. I have had some unpleasant circumstances with HR as well and the 'private' information did find its way back to my boss.
Do you have an employee relations department at your job or something similar where you can go to filre a complaint against co-workers?
Please keep us informed. Good luck!