"I would love all and any of you to post pics of you and her and/or memories you have of her down in the comments," he wrote in the post. "I will cherish it forever and always. So please do that for me."
He also took the opportunity to share his feelings about his beloved sister.
"Christina was more than my sister," he said. "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 ill do without her."
The 22-year-old singer was fatally shot late Friday after opening for pop-rock band Before You Exit at the Plaza Live venue in Orlando, Florida.
At a press conference Saturday morning, Orlando police chief John Mina said Grimmie was possibly shot by a deranged fan, who then took his own life after being tackled by Marcus. Saturday afternoon, Orlando police identified the shooter as 27-year-old Kevin James Loibl.
Marcus was hailed as a hero by police for his swift action.
"Very heroic actions by Marcus to jump in, and definitely could have prevented further loss of life," Orlando police chief John Mina said on Saturday morning.