Sounds like she got drunk and let him smashed raw.
This whole story doesn't make any sense, OP should've called her Mom,sis and everyone else he knew at that party to see what went down.
Why would a guy who tried to hit on her in the past randomly showed up at her sister's party/gathering ?
She told him to come there.
OP use your common sense, if she's in another state she's going to cheat.