Marie-Christine is hard to spot because she has dyed her hair brown/red! She and her husband are in the back row of the group picture between Kathleen and Stephanie. She is wearing a dark blue V-neck gown. (See photo 17/24)
You can also see a picture of her bringing up the rear of the children who were wedding attendants. One of her sons is the older of the two wearing grey Austrian outfits. (See photo 20/24)
It seems that M-C was a witness for the couple, as she is sitting behind and to the right of the altar during the ceremony. (Hard to see on photo 6/24)
Regarding the red gowns, Marie-Astrid is wearing the same red dress that she wore to the wedding of her niece, Princess Marie-Gabrielle de Nassau, on September 2, 2017. She has worn other repeat outfits to all of her children's weddings, including one belonging to her mother. She is the "Princess Anne of Luxembourg," in that regard.