User blog comment:B214/Volume 23 discussion blog/@comment-28087053-20170318185110/@comment-10330822-20170319020718

Yeah, and even saying that I feel he probably still wouldn't do it. Even if Grayfia made a pass at him and Sirzechs gave the thumbs up for some stupid reason, I feel Issei may still find it weird. Issei does seem to have some boundries, the operative word being "some". The fact he views Sirzechs and Grayfia like siblings might make it too weird even for him. Similar to how Yasaka made a pass at him even though she has a daughter not that much younger than Issei, Issei seemed kind of disturbed by it.