My opinion is that by taking offence at this the OP and her husband will be cutting off their noses to spite their faces. Yrs, it would have been nice to know that your SiL was getting married, you could have sent her your best wishes and perhaps a gift - but when and where she marries is entirely her own business. If she or her new husband have been married before, they will have one all the big do stuff once and may not have fancied all the hoo-ha a second time round. In any case, not everyone wants to have a big wedding party. They may feel that they just want to be on their own and make their vows.
Then think of who else they would have had to ask if they asked you - more family, friends of each of them, partners and children of those friends and family, it soon adds up to large numbers and a large bill.
If you want to celebrate with them, invite them out for a nice meal in a few weeks time, just the four of you, and wish them joy. And keep quiet about not being invited, reproaches are not a good way to celebrate.