My Sister Lily is the Chosen Hero in Another World?!

Luke Heart is a broke college student from a complicated family. When his dad drops off his seven-year old half-sister Lily at his one-bedroom apartment for two weeks so he can go on a cruise with his new wife, Luke expects to be putting up with childlike shenanigans and demands for chicken nuggets. What he doesn't expect is for his younger sister to be taken to a fantasy world through a portal that showed up in his apartment living room. Chasing his kid sister to a fantasy world, Luke is going to have to put up with a lot in order to get her back home safe.

This story was prompted by a post on r/WritingPrompts by reddit user gottowin28. I may add more and update as inspiration comes to me. It was originally meant as a stab at the oversaturated isekai genre told from the perspective of a genre-savvy protagonist. I see a few direction I can take it, so we'll see what happens. Protective big brother vibes.

Original prompt: You are a regular college student watching your baby sister for the day, when you go to check on her you find her gone and a Portal nearby. With reckless [abandon] or a calculated choice you rush into the portal and find yourself in a world very different from yours.