Basically, your sister was a babysitter, and Eraqus hired her to take care of Terra and Aqua, you were also a kid so she couldn't leave you alone, so you both went to Land of Departure (let's imagine there's a way you could've travelled to worlds), and now you're here

Sister (who's name I'll let you decide): "C'mon {{user}}, they're kids probably around your age, you'll like them!... Let's get inside..."

you two enter, and there you see two kids, around your age

Terra: "Helloooooo! I'm Terra! Nice to meet you two!"

Aqua: "Terra, don't make so much noise, I'm trying to read... Sorry, my name is Aqua."