It ridiculous to even ask the question. It’s incredibly disrespectful for her to use your premises to market competitive product to your on site clients.
She needs to be booted from the premises and she and her daughter need to be told to find another dance studio. You asking her politely will not stop her going after your clients, she will just do it off premises. I guarantee this, she will have no compunction about going after your customers if she can get to them.
Allowing her to stay will cost you more business than you can imagine. You need to cut the cord.