Hmmmm, sounds a lot like my first job right out of high school. My bosses father had a choke hold on the family business. He would come in and basically screw everything up, and his son let it happen for some time. Finally, my boss told his dad to stay away.... he showed him the books, money was better than ever before, and him coming in and ruining everyone's mood was killing everyone.....
I'd suggest what specialmom did. Ask your boss about who is in charge, and then ask him (politely) to do something to remedy the situation.
Why don't you ask her son how to handle things. You don't have to complain about her but ask him what the best way to deal with her would be. May be time to look for new employment. good luck
Is the owner your boss or is his mom your boss? If you aren't sure, ask the owner. If he says "I'm your boss," then say to him that you are having a problem with his mother interfering with your ability to do your work in a professional manner. Use language like "I am trying to maintain a professional tone and she tells me that I am expected to prevent the therapists from accessing the desk when they need something," etc. etc. In other words, explain what happened without emotion, as though your entire and only goal is to do a professional-quality job. Don't use language like "she expects me to start trouble with the therapists," since that can't be quantified and it does not describe behavior anyway, just an implication that you came up with. He needs specifics and how it gets in the way of what you feel is professional behavior by yourself and the therapists. Explain that you want to do your job in a way that reflects positively on the establishment and helps to give it a good reputation, but you feel you can't do it well when she is giving you instructions. Then see if he says, "Well, she's my mom, you have to do what she says." If so, give two week's notice.