I remember this oneshot that I read that was included as an unrelated story in a larger series. I don't remember what series it was, though.

It was about this high school(?) boy who recently got a new younger stepsibling, who was like a preschooler or elementary schooler. At the beginning the boy didn't get along with his stepsibling, but by the end of it he turns into a doting older brother. Also he thought he had a stepbrother, but it turned out to be a little stepsister. The boy was also on the baseball team at his school I think. The boy had a feminine sounding name and his stepsister had a name that sounded like a boy's.

I've been trying to look for this for a long time. Thanks to anyone who finds it!

I'm pretty sure it's Mizutama Honey Boy volume 2 special one-shot: "Yoroshiku Brother?"