Do I have to have a certain realtor?

My fiance and I are looking to purchase a new home. We just sold our current home and had called the woman who has this new house listed. NEW house therefore no previous owners ;) We came to the open house and also asked to see it a second time after that. NOW we are serious about buying it and wanted a relative to be our agent. Now we hear that only the woman who showed it to us can make the commission. We never asked her or signed anything for her to be our realtor, only for her to show us HER listing a second time. We live in Wisconsin I don't know if that helps....but does this sound correct? I'm baffled at how she just magically becomes our realtor because frankly...I just don't like her!!