Can't wait to see this be made
What does "younger sister" mean to you? In official version this would mean 18 or 21 with the protagonist being somewhere around his mid-twenties. But because this story will most likely be modded, it can also mean an age of 12.
Let the one who create it decide I would say, unless you want to edit their story then it would be even better, That what i would totally do.
Since there modding, no need to sweat the small stuff.