It looks to me as though the water is a little murky here.
This is not legal advice. For legal advice ask an attorney.
In real estate there is term called "procuring cause". How is did it come to pass that the agent that had the buyers knew about your home being on the market?
In most cases the reason they knew is due to the agent's marketing. Legally the question boils down to the fact the if they found out based on the agent's marketing, then you owe them a commission.
You should also know that most brokers know (the people for whom the agent works) that is it not uncommon for buyers to wait until the listing expires to contact the sellers directly, hoping to "save the cost of the commission". Because you are dealing with people's livelihoods (to you it is a commission, to the Realtors it is how they earn a living), I would not be surprised IF you opted to not use your agent, the broker would let the transaction close, then sue you for performance (meaning they would prove, and probably could do so, that it was due to their agent's marketing activity that the buyers were aware of the property. So your relative may not take action , but their broker probably would.
The fact that they never offered you a copy of the listing contract concerns me Did you ask them for a copy?