Why do you think your sister is negative? What kind of things is she negative about?

You could suggest she see a therapist or read some self-help books.

If she doesn't think she has a problem..there's not much you can do. Some people just are negative and prefer to be that way. Closed-mindedness is a way of protecting oneself. She might have many fears and takes it out on people ( or herself) in a negative way. Maybe she feels defeated or hopeless. Maybe she doesn't want to be happy or doesn't think she deserves it?

Another option is for you to do research on negative attitudes. If you understand it better, then you'd have some insight as to what questions to ask her or what things to say to encourage her.

Remember, some people are negative because they get a payoff from people always trying to encourage them and give them positive reinforcement. It's a dysfunctional method and can become a habit where they don't have to make any changes to get the attention they may need.

Change requires a lot of hard work....and the first step is realising and admitting change is needed. That can be quite easily rejected with strong negative emotions focussed in another's direction.

Self-placed emotional armour is not so easily penetrated.

Best to you both,
quincy