Your sister had been with her partner for some time and has children with him. When she left him briefly last year, she came to you.
Is there a possibility that she felt you might voice some opposition or caution to her marrying him and that may be one of the reasons she and her now husband chose not to announce their plans in advance to anyone? And if there is a possibility that his side of the family would have been problematic, can you not just sit with that. I suspect there is a reason they chose to tell no-one in advance.
Forty years ago, if my late husband and I had told his mother that we were expecting a planned baby and a small wedding before the baby was born, we knew from experience that her reaction would have been: This is the wrong time, the most important person at the moment is your sister and her needs.
In fact, that is exactly what she did say when she was told the day after after we got married - "It's not the right time." Truth is, it would never have been "the right time" for her because her life revolved around her daughter.
It seems that over the years the concept of "elopement" has changed. It used to be a term that meant that a couple, often a very young couple, would travel to another part of the country to get married without their parents' knowledge, consent or blessing. Or perhaps where one party was a divorcee or there was a scandal surrounding the relationship. People now seem to be using the term for couples of any age who get married quietly with a small number of guests or who book a registry office wedding with just witnesses in attendance or go abroad for their wedding without telling others in advance.