Yeah, highly doubtful that ISIS will say "Umm, okay, if you're sure you want to, you can leave." Their lives were forfeit (one way or another) the minute they arrive in Syria. I doubt that Austria has the capability to execute a snatch and grab raid to bring them home.
As the father of a daughter, I truly feel for the parents. One is only 15. (The 17 YO looks shocking similar to my girlfriend when we were that age.) If word gets back to their "husbands" that they wish to leave, well. It's probably too much to hope that their bodies would be returned to Austria.