"Happy Birthday A!" Gigi, 20, posted to Instagram. "Thanks for lovin my sis like you do. You have so much to celebrate tonight, beyond proud of you! XO."
The R&B singer is dating Gigi's 19-year-old sister, Bella, and the couple made their red carpet debut at the awards show on Monday. Bella wore a black dress with a plunging neckline, while the "Can't Feel My Face" singer matched her in a dark tux.
Bella also posted a sweet birthday message Tuesday, writing, "Happy birthday to the most humble, kind, hard working man I've ever known! 1 year and you're still my favorite person on the planet. You deserve everything great in life."
On Monday, Bella posted a message to her boyfriend before his GRAMMYs performance, writing, "This is your day baby! I'm so proud of you. I feel so lucky to be by your side."
While The Weeknd seems to be having the time of his life, this has been a big week for Gigi as well. The Sports Illustrated Swimsuit model put her ludicrous body shamers to shame inside the pages of this year's issue.
Check out the three gorgeous 2016 Sports Illustrated Swimsuit Issue covers in the video below.