Huh?! Most people only have one bathroom, not sure how you think people in other parts of the world live, but the most bathrooms I have seen in a home is two, but that is rare. I grew up with two brothers, a sister and a mother and we never used the bathroom at the same time, clothed or unclothed (definitely not clothed). Do so is not normal. You generally waited your turn. If you had to "go badly" and someone was in the bathroom, maybe in the shower you could slip in, do your business and leave quickly, but that was rare.
With that said, looking for realism in an adult visual novel is a bit of a stretch in my humble opinion, though siblings being totally nude together in the bathroom is stretching it a tad unless they're already sexually attracted to each other which, trust me, does NOT come naturally... I have a little sister, she was annoying growing up. She DID flash me her boobs one time after she got pregnant and said "what's the matter, don't like your little sister?" which grossed me out (we liked to do that, we had a joke "incest is best!" but were NEVER attracted like that, just an ongoing joke). Now that she's older I can "gross her out" with incest jokes.