Unbalance

The Hungry Bear is a restaurant where carnal appetites are satisfied to order. The waitress, Mika, has just uncovered the secret life of her late sister, a woman who would perform any sexual act, no matter how bizarre! But the secret’s all in the sauce, and Mika vows to service her clientele’s insatiable appetite for the exotic!

[From AniDB]

The story revolves round a rich man's son, whose wife, Misato, died a year earlier in a traffic accident. After her death, the main character can only indulge himself in drinking. By chance, Misato's little sister, Mika, had a crush on him and one spring night, she could not stand to look at the main character wasting his life away on drinking, so she confessed her love to him in hope of taking over Misato's place in his life.
As a result Mika sort of becomes a sex slave when the main story starts six months later, but it seems that the main character has not gotten over the death of Misato and he's quite bitter about it...

[From animedensetsu]