Kyuuketsu Gajin

Having lost their parents in a traffic accident, the siblings Kentarou and Kyouko live happily together in their mansion. Kentarou loves insects and, like his father, aspires to become a scientist. He became a research scientist as he had hoped and married the beautiful and kind Yuuko. The three of them lived happily, but a shadow began to fall over their happiness when he started a certain research project.

(Source: Kaiki Manga no Anagura, translated)