Hi Jeff, congratulations on your engagement and welcome.
You sound exactly like my husband when we were getting married. My family made the "threats" too and wound up boycotting our wedding and reception. He had over 90 guests and I had 10. It was an awesome time none the less and we're still happily married 9 years later without my family's blessing. I think my family's bad behavior has done nothing but bring me closer to my husband's family. I don't think many people have close relationships with their in-laws like I do.
Her family sounds suprisingly liberal for JW's to me the fact that they've had peacful contact thus far is rare. She is luckier than most in that respect. Your wife is right, to them she's the one who's turned her back on them by rejecting their belief system. It's obvious to anyone who's never been a part of their mindset that the opposite is in fact true but changing their minds on this is a daunting task at best and impossible at worst.
You should just enjoy your special day and if they show up fine, if not don't let it ruin what should be one of the happiest days of your lives.