Hi! I am looking for a manga with a plot like this. The girl's older brother passed away and she suddenly met an angel on his grave. He said that he was sent as a guardian by her late brother from heaven. To cut the story short, she fell in love with that angel. The last scene was when he left her and then she suddenly saw him in uniform in her brother's grave. It turns out that he was a friend of her brother and was asked to take care of his sister.

Usotsuki Tenshi to Ama no Jaku
It's a oneshot though