Just started dating this girl?

I just started seeing this girl about two weeks ago. We have been friends for quite some time before this, but moreso 7 years ago. We kinda bumped into each other and were single. Anyways... She just told me she was thinking about dating this guy before we bumped into each other. She found out that this guy was her stepmom's sisters son. So she is sort of related to him by marriage, as a step whatever. She just told me that they have plans to spend the weekend at one of our local casino's in a couple of weeks. Now in her eyes she sees him as a relative all of a sudden, but will not budge when I tell her going with him is innapropriate. Am I wrong?