OP, your sister evidently had her reasons for choosing the type of wedding she had.
If she had told you what her plans were, then she would have had to decide whether or not she also told your mum.
Perhaps she worried that if she did tell your mother there might be pressure from your mum to have a family wedding.
Perhaps she also felt that if you knew, she would be placing a burden of secrecy on you and that afterwards, your mum might have been upset that you knew about your sister's plans but she did not.
For a number of reasons, I had a semi-secret registry office wedding with two friends as witnesses; we were 34 and 40 and expecting a planned baby.
My side of the family knew what our plans were and knew which day we would be getting married but my late husband's side of the family (a bedbound grandmother, a difficult mother who would not leave the house because my husband's sister, who had never left home, was in the throws of another mental health crisis) were not told in advance but told the day after.
The day after our wedding, my husband spent much of the day driving his mother to a psych hospital to collect his sister who had voluntarily admitted herself on the day we got wed but then decided she did not like being on a psych ward and wanted to be fetched home. In the midst of all this, we told his mum that we had some good news to share that a) we got married at a registry office, yesterday and b) we are expecting a baby in May - so you'll be a grandma. Her reaction to this news was: "Oh, it's not the right time."
So, if we had decided to invite both sides of the family, his sister would not have come as she was having a mental health crisis; his mother would not have come because she would not leave his sister in the house on her own, and the grandma was bedbound anyway. As it was, the first day after our marriage was spent sorting out his family's problems.
On my side of the family, my mother had initially said she understood the situation with my late husband's family and why we were thinking about not having any quests. Then a week or two later, she said if you need help with wedding expenses, we can help you. Then she told me she had bought an outfit - even though she knew that we were not planning to have quests.
Then my late sister said that every wedding that she and her husband had been to every single couple had since split up and she and my brother-in-law were not prepared to go to any more weddings. (So that meant that if we had decided on a small family gathering, not only would my husband's family not have attended but my sister and her family would not have come, either.)
The day after our wedding, I phoned my sister to thank her for a JL gift voucher she had sent and how our plans to spend the day had been scuppered by his family's problems and my brother-in-law, said: You do realise that your mother was very upset about not being able to attend your wedding and was threatening to pitch up at the registry office, anyway, and we had to persuade her not to? Which I would rather have not known about, especially as she had a sent a very nice Inter Flora flower arrangement to us on the morning we were getting married.
So it was all a bit of a mess. But I don't regret doing it that way.
Just ask your sister gently why she decided not to tell anyone.