You look around, and get an idea. It's disgusting, but it just might work: Lexi's stomach is full of cereal and milk right now, and is using almost lots of strength to move it all around, getting it nice and digested. You figure that, with a bit of an extra nuisance, Lexi's stomach will start to hurt, and she'll eventually end up throwing up, freeing you of this humid cave.
Spotting the nearest wall moving in to push some half-chewed cereal around, you decide to begin and violently jump up and down instead of walking to reach it. You may be small, but even that causes the huge pool of stomach acid and breakfast to shake a little. Good start. When you reach the stomach wall, you start to punch and kick at it with all your strength, keeping very careful to not fall over and be submerged in the mixture below you...you don't want to drown inside your sister, after all.
Outside, your irritation is already affecting the 12-year-old girl. She winces a bit in pain as her stomach emits a loud, deep gurgling noise.
"Ow, why does my stomach hurt...?" She mutters to herself, then remembers her extra snack, "...Stupid brother." Lexi rubs her belly, hoping that will soothe it and allow you to become food easier.
She may have figured out the source of her discomfort, but of course she doesn't know that you physically cannot digest. You go around, spreading your jumping-punching-kicking torment around the entirety of your sister's stomach. Finally, you are starting to see some results. The pool of food and acid is now tumbling like a stormy sea. You can feel bubbles of air whooshing up past you, cementing the first step in the process.
Lexi goes to lay down, as her stomach pain is only getting worse. Before she can, though, she is surprised by an upward rush of air, and lets out a loud burp. Unlike the satisfying belch after eating, this one doesn't feel as good.
"Ooh, my stomach..." She lays down, shaking up the contents even more. They (and by consequence you) settle back down in the new gravity, and you swim up to a drier area. Lexi continues to release frequent tiny burps, as you press on with making her sick.
Suddenly, what you wished for finally begins. Lexi lets out a terribly deep, powerful burp. "Ugh, I'm gonna throw up..." She cries, heading to the bathroom.