Fabio is ineligible to play against United under the terms of his loan deal but has conceded that watching from the stands was a strange experience.
He told Globoesporte: "I was pleased to see my brother score a goal like that. Naturally I wanted to celebrate. But I was sat next to the QPR president."
"I told him afterwards: 'You never score goals like that! You had to do it against my team', but I was happy for him.
"I've grown a lot this season. We've both matured. He's flying [for Manchester United] and I'm playing more. The change has been good for us."
Rafael has nailed down a first-team place at Old Trafford this season, while Fabio has made 17 appearances for the Hoops.