So, I think I am looking for a shoujo manga.

It's been quite a while since I read that one so I am not sure if I remember everything correctly.

The manga is about a girl whose sister died because of an illness. Obivously she was very sad and shocked that's why she constantly cried. At first the people in her school were understanding of her situation but after a while they just found it annoying that she kept crying because everything reminded her of her sister. She either changed schools or went to high school.
In the new school she found new friends and a boy that she fell in love. I think he helped her come to terms with the death of her sister.

I also remember a part where she regretted not being able to properly say goodbye to her sister because she cried the whole time and told her sister not to leave her.