Agent Representation Question

Hello All,

I have an investment property that I am looking to sell. I connected with a local agent and discussed price, marketing, etc. I entered into a listing agreement with the agent.  

Shortly afterwards, the agent advised that her sister was interested in purchasing the property and that she would be an unrepresented buyer. The sister submitted an offer and the contract included the personal interest disclosure nothing the sister. However, the contract also listed the sister and my agent on the deed.  

I raised a concern about whether I could be properly represented in such a transaction.  The agent's broker suggested that my agent become a facilitator or that I myself am shifted to an unrepresented seller.  

I'd appreciate any thoughts on how I can protect myself as a Seller in this scenario. Thank you