Your sister is lucky to have a family member who is concerned about her as you are.

Sometimes people can be in denial that anything is wrong with them.

You can take a horse to water but you can't make it drink. You might want to gently suggest perhaps she sees someone but ultimately that's up to her. Only a professional will be able to shed some possible insight.

From your objective opinion you believe she displays the symptoms. I wonder what her opinion would be. Sometimes our opinions of our behavior are different to the ones who love us and she may, or may not, view things the same way that you do.