Advice needed, please

[SATX*APhi]

One of my sisters (A) has a class with this guy named Jack who she is attracted to. Thursday night she went out with one of her friends (F) to a bar and ran into him. Well (A) starts talking to Jack. At the end of the night (A) & (F) (not Abercrombie and Fitch) went over to Jacks for a while. (A) and Jack really hit it off and he tells her that he really likes her. The next evening, Friday, (A) heads over to Jacks to hang out. There is a knock at the door and it is (S), another sister. (S) did not know that (A) was there because she never saw her. The next day (S) told Jack that she knew that he had a visitor when she went over. Here is the problem: When (A) was first interested in Jack, (F) told (A) that another of our sisters (S) cheated on her boyfriend with Jack. From what (A) hears, (S) still likes Jack, but is still with her boyfriend. (A) really, really likes Jack but does not know what to do, and is afraid (S) will hate her.

When someone comes to me with a problem, I am an excellent listener -- I just really suck when it comes to giving advice! She knows of GC and I told her I would ask all of you your opinion about how she should handle the situation. Advice about the situation would be appreciated. Thanks!