Accompanied by boyfriend David Lucado and sons Jayden James and Sean, the 32-year-old singer wore a silver gown to the Audubon Tea Room nuptials between Jamie Lynn Spears, 22, and her 31-year-old fiancé James Watson.
Jamie Lynn has a 5-year-old daughter Maddie Briann with former fiancé Casey Aldridge. The one-time Nickelodeon star made a splash in 2008 when she announced she was pregnant.
She and Aldridge ended their engagement in 2010.
Jamie Lynn told social media followers last year that Watson had proposed.
"Yesterday was magical :)" Britney tweeted on Saturday. "So happy I could be a part of my little sister’s big day. Congrats @jamielynnspears!!"