Bro/sis complex in jp manga/anime

I was reading high school debut and yoh’s sister just fucking weirded me out. It makes me so fucking uncomfortable when authors write siblings who have a bro/sis complex. I can never understand how people can feel that way. And the worst thing is I know people who feel that way irl. Like I know this one girl who said she’d be jealous when her brother gets married and would hate to “share” him with another woman. LIKE WHAT?????