This Father-In-Law Lives Alone And Is Pretending To Be Senile, But The Truth Is That He's A Horny Sexual Monster Who Is Disguised As A Harmless Old Man. Tsubaki Sannomiya

ボケたフリした独り身のお義父さんは、性欲旺盛な老人の皮を被った絶倫モンスターでした。 三宮つばき

Also known as SSIS-109

Tsubaki Sannomiya agrees to let her father-in-law move in with the couple, because of his growing dementia. However, the father-in-law is not as gone as Tsubaki believes, as everything is part of a scheme. Eventually, he attempts to grope her and she let the incident slide. To preserve the peace at the house. Tough mistake.