Suna No Shiro

It's about a rich man's daughter and a orphan boy growing up together and falling in love. But when her parents die, her aunt wants to send the boy away to college and make her marry a respectful man to save the family reputation.
[From mangatraders]
In a quiet area of France during WWII, a very young orphan boy was left on the steps of a large mansion on the same day that a daughter was born to the bourgeois family living there. From that day on, Francis and Natalie grew up together and found themselves falling deeply in love. As talk of their marriage began to stir, Natalie suddenly lost her parents to a plane crash, leaving her aunt as her guardian. This aunt, vehemently opposed to Natalie's engagement to a mere orphan, which drove Natalie and Francis to elope. Unfortunately, they were caught. Rather than be torn apart from each other, Natalie and Francis decide to spend eternity together by jumping off a cliff. Through a cruel twist of fate, however, only Natalie survives...Is there even a smidgen of hope that Natalie will somehow be able to find her love once again? An amazingly well-developed, emotional work of art from Ichijo Yukari.