I can't work out from your description whether other family members and friends were invited to the wedding, or whether they only had the legal witnesses present.
I can understand only too well why you feel hurt, but if no-one was invited except witnesses I feel you have less reason to be upset, than if you and your husband were excluded.
However, if you want a good relationship with your sister-in-law and her new husband, I think the best thing is to send your congrats and say nothing about your hurt, right now. Perhaps later, you can at some time mention that you were sad not to be there and felt excluded.
A young woman who we practically regard as a daughter did the same to us, after having invited us to her wedding, she then got married without telling anyone and only sent a text message afterwards! We were badly hurt and made, what I now see was the mistake of saying so, so now we are barely on speaking terms. In hindsight I would have done better to have held my tongue, as we both miss her.