03/06/2014 at 05:00 PM EST
Alexis Shapiro grips her mom Jenny's arm as she slowly makes her way to a dock along the San Antonio River.
"It hurts to walk," says Alexis, a men's XXL T-shirt stretched over her belly.
Though she's been begging for days to go on a boat ride near the family's Cibolo, Texas, home, Alexis stops to whisper something into her mother's ear.
Jenny, 34, doesn't flinch as she looks her daughter straight in the eye to reassure her.
"Don't worry, this boat can hold a lot of people." With that, the 203-lb. 12-year-old gingerly boards the vessel with her brother Ethan, 8, and Jenny.
As Alexis waves to her dad, Ian, 34, and sister Kayley, 9, who have remained ashore, Ethan quietly asks his mom, "What if the life vest doesn't fit her?"
On a rare day out, it's clear that her family's support is what truly keeps Alexis afloat. She suffers from hypothalamic obesity – an irreversible condition that causes insatiable hunger, a slowed metabolism and uncontrollable weight gain.