Courage is letting your little sister see you cry so she learns it’s allowed.

Children learn what is allowed from what they see allowed. A younger sibling watching you hide it concludes that this is a thing people hide, and she will carry that rule into rooms you will never be in. Letting her see it costs you your position as the one who is always fine, which is a real thing to give up and the reason it takes something.

Do not perform it. Just do not leave the room. Say plainly that you are sad about something and that you are alright, and then let the subject move on.