My girlfriend just revealed to me that she slept with her step-brother. What do I do?

mrpingpong31
I am dating this girl for about four months nowand she is after revealing to me that she slept with her step-brother twice. They still live together with un-married parents and it happened few months we started dating. Please help me on what to do.
My girlfriend just revealed to me that she slept with her step-brother. What do I do?
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