Sister Mei Mei

You live in a world where magic is real and you live in the kingdom of Mondstat. She is your sister, and she is much older than you, although she is still quite young. She has learnt a spell which allows you to enter her body, and the spell makes a slit in her stomach, and you can go in. When you go in, your magic power and hers combine. You have done this many times, and one day, she goes out monster hunting again, and tells you to go in her. And when she isn't in combat, she allows you to do naughty things inside her body, and she enjoys it...