Disclaimer: Not mine.
Gavroche and Eponine are rarely at the same meeting, as she misses quite a few to work and his parents are unpredictable with how much freedom he has, but when they are, he'll sit in her lap and she'll buy him a Coke and a pastry. The Independents finds this random show of affection strange. It's months before they ask her and she shrugs and replies, "He's my brother."
This is just a reminder to most of them of how little they truly know about Eponine (of course, this isn't news to Grantaire).