"Christina was more than my sister," her brother, who goes by Mark, wrote on Facebook. "She was a partner in life. A superstar. A goofball. Introverted. And a friend to everyone. Genuinely. But above all...she was my baby sister. She loved the Lord and her family and was always there for me and I honestly don't know what I'll do without her."
Christina's friend Ashly Nicole told E! News the singer and her brother were "as close as can be."
"He was the perfect brother to her and he became almost like a big brother to me too," she said. "They were only a year apart and he would always talk about how he would do anything for her. She was his world. He cared about her so much."
A source told E! News Christina's brother was her best friend. The two even had "Player 1" and "Player 2" tattoos on their arms. The source said Christina was also extremely close with her father and mother, who has been battling cancer.
Marcus also often performed with Christina and helped her out with her YouTube videos. He had accompanied her on her recent tour with the pop punk band Before You Exit. She had opened for the group at Friday's concert.
In his Facebook post, her brother also thanked the scores of fans who have posted prayers and other messages of support on social media.
"The support has been crazy and I do feel at peace," he wrote. "I just don't have any words. I love you all."