Omoi, Tokihanatareta Toki

Main character is a sophomore student. His father remarries, and protagonist's classmate girl becomes his new sister. With new mother, new sister and a cute childhood friend his summer is approaching it height.

[From vndbreview]

The main interesting feature of this game is that the new stepmother/oneesan character is customisable in many ways - hairstyle, haircolor, breastsize, tan pattern, etc.