Its normal. Even sleeping in the same bed is not out of the ordinary among siblings. Many of my guy friends would sleep in the same bed with their sisters, granted they were usually Asian or Hispanic.

There’s definitely a bias here. There are many siblings that have close bonds that are scolded at by outsiders, because it’s not understood.

If their relationship is truly bothersome, you can always broach the topic with a curious tone. Ask your boyfriend a question or two about his relationship with his sister as if you’re curious and like to learn about their sibling dynamics. Act like you have no clue that what’s going on is weird, and that you’re interested in learning more. That way, there won’t be any foul play.