Ok...an answer to the original question.
Since the woman has never met the Gorgeous Man, he is not a "known" member of the family nor is he married or involved with any family member.
Since they did not exchange phone numbers, it can be assumed the woman had no continuing interest in the man.
The man, however, must have an interest in the deceased and that would be what the woman would have naturally found out.
This information would be what caused the woman to kill her sister.
This could be ...the man is at the funeral because the deceased is his father. The deceased is also the husband of the woman. Further, the man identified the sister as his mother.
Since the woman did not know the man, it is assumed that he was not the child of the husband and the sister from an acceptable relationship prior to the woman and the husband's marriage.
Therefore, the man is the result of an affair between the husband and the sister, the motive for the killing.