Billie Eilish Officiates Brother Finneas’s Wedding to Claudia Sulewski

The "Ocean Eyes" singer traded the stage for the altar, guiding her older brother through his vows in front of an A-list crowd.

Billie Eilish assumed an important role at her brother Finneas's wedding to his longtime partner, Claudia Sulewski. The couple tied the knot in a star-studded ceremony on Saturday, August 29, in Montecito, California.

In photos from the outdoor nuptials, Billie can be seen smiling as she officiated the wedding in front of loved ones, including a slate of celebrity friends such as Justin and Hailey Bieber, Kristen Bell, Dax Shepard, and Jason Sudeikis.

The "Happier Than Ever" singer stood at the center of it all in an off-the-shoulder, sage-green gown with a fitted, corset-style bodice and a sweetheart neckline.

The dress featured structured sleeves and a soft, flowing skirt. She wore her hair slicked back at the roots with bumped ends, keeping her look simple and understated as she read from a large book in her hands.

Beside her, the bride stunned in a strapless white ball gown with a fitted, structured bodice and a full skirt made up of voluminous layers of fabric, paired with a cathedral-length veil that trailed gracefully across the grass.

Finneas, meanwhile, matched the occasion in a classic black tuxedo with a coordinating bow tie, wearing his long hair down in a middle part. The singer announced his engagement to Claudia last September, after seven years together.