The Testament of Sister New Devil
Warning: The following anime are NSFW, much like High School DxD itself.
The Testament of Sister New Devil manga has recently been remade (The Testament of Sister New Devil STORM!), but the original’s greater emphasis on action over sex makes it feel a lot more like High School DxD. It’s this version that has an anime, and it’s a pretty good one too.
Basara Tōjō, a high school student, gets two new stepsisters when his dad remarries: Mio and Maria. Basara soon learns that his two stepsisters are actually of the demon clan. Maria is a succubus and servant to Mio, the Demon Princess and next in line to rule. They attempt to enslave Basara, only to learn that Basara comes from the hero clan, and possesses a cursed sword specifically crafted to kill demons. The trio of step-siblings form a truce, and attempt to use a spell to make it easier for Basara to look out for his new baby stepsisters, but Maria messes it up. The spell ends up cursing Mio, and forces her to experience an extreme aphrodisiac effect whenever she disobeys Basara. The three siblings must learn to live together and accept their new status quo, all while Basara and Maria work to protect Mio from other demons that covet her title for the throne.
Like High School DxD, the main character of The Testament of Sister New Devil is surrounded by devils. Much like Issei, Basara doesn’t view them as demonic though. To Basara, Mio and Maria are his sisters, and nothing more. This makes Basara and Mio’s growing sexual relationship feel more than a little like incest, but much like the sexual encounters in High School DxD. These moments add both erotic and comedic elements to the story and help spice up the smaller scenes between the fantastical battles.