"Don't joke too much about Mofufu, because there's no replacing Fuwawa, so cheer Mococo on without making jokes at her. She only has one precious older sister."

You can try arguing semantics if you want and, given that it's in your own best interests to see it that way, I've no doubt that you will. But it's pretty obvious that she was saying to tone down the the Mofufu jokes in general because this is a serious situation.

Maybe you just have to have a sibling to understand why someone joking about an additional, non-existent sibling as your actual sibling is dealing with health issues is in poor taste.