Trying to find this kdrama, I only saw a short trailer, and I thought I bookmarked it but can't find it.

It's about a young woman only around 18 or 19 still in school. Unexpectedly her older sister dies and leaves behind a daughter about 5-6 years old. So she has to raise her and become her mom.

The young aunt has to give up on her dreams, a future in sports where I think she was doing some sports in school, can't remember what sport.

I think it's a very recent kdrama, 2022 or 2023 even.

By your description, it must be "Crash course in romance" which is an ongoing drama.

Ok, from short trailer (actually a shorts) it was a different kind of drama, looks like this story I described plays only a minor role really. But yes I think that's the right drama.