I'm going to tell my sister to get over it
Do you really think she really needs to get over it?
Yes, I think she should save her tears for a rainy day!
I can see how annoying it can be when your sibling is always moping, but they do it for good reason. It's not your place to be rude about it and be so dismissive, they need your support and advice, not getting shunned. They may feel so alone when you don't understand.
I don't get it, she keeps doing it all the time.
Have you ever tried talking to her why she might be sad? You might get to know her better and grow a closer bond with her. She might feel depressed if she is never taken seriously.