Jashin no Mago - Dark Elf Shimai to Sugosu Isekai Hikikomori Seikatsu

Satoshi Katakura, living a shut-in life, earns money through investments using the inheritance from his grandmother. One day, he wakes up to find that an otherworldly realm has spread outside his home. Appearing before him are dark elf sisters who revere his grandmother as the Evil God. "From this day forward, we pledge our eternal loyalty to you as our master!" What is the sisters' true purpose? What is the real identity of his grandmother?