I hope you feel calmer about this now, Sleepyamber. You weren't excluded from the wedding, as you put it - your sister in law and new husband just had the quiet wedding they wanted, and you should be happy for them. Too many people are pressurised into having the wedding day they feel other people want, instead of doing what is best for them.
Our children have run the gamut, from a huge castle do, a private house ceremony followed by a big party in a barn, a stylish town hall wedding and meal for 16, and a registry office ceremony for just the happy couple and one friend each as witnesses. We were genuinely happy for each of them, we gave them the same wedding fund to spend on the day or their house or whatever they chose, and their weddings reflected their personalities and made them happy.
Don't whatever you do let your resentment spoil their happiness or your own. Life's far too short for that.