Siblings can say things to each other nobody else is allowed to. That privilege still has a line.

Closeness buys you a wider range, not an unlimited one. You can mock each other in ways that would end a friendship, because you both know the history behind it and neither of you is in any real doubt. The line sits where the joke stops being shared and starts being aimed, and both of you know when it got crossed.

If the room went quiet after something you said, that was the line. Say so directly instead of hiding behind how the two of you always talk. Having one subject you have agreed not to touch is a sign the thing is solid, not fragile.