I must admit this is what my second husband and I did. He is now my ex, but that's beside the point 
We were both over 40, I had been married before and neither of us wanted a huge fuss. Family were all wide spread, so trying to get everyone together at the same time would have been almost impossible.
We went off for a long weekend and married. No family at all were there. Our witness's were people who worked in the registry office. Our photographer was the gardener there.
Family were a little surprised when we told them, and probably did make comments to each other about not being invited, though they never said anything to us.
It was our day to do as we wanted. I think that you are being unreasonable to not support your sister in law. Just be happy for the new couple.