Congratulations on turning 30, what an amazing decade you have ahead of you! It is the decade in which you learn to become comfortable with yourself. And that's exactly what you want, right?
I completely agree with royaltywriter. You need to focus on YOU. Your sister is your sister. I don't have a sister but I have seen many times people checking out women whom they perceive as looking better than me. It did use to bother me until I came to my senses in my 30s, seriously. I decided that I will carry myself the way I really like carrying myself, without comparing myself to anyone else. Because, believe me, even when you don;t have a sister, you will find someone to compare yourself to. It's only natural. And because you grew up together in your case, it is also very normal to feel jealous. So congrats from me as well, for being so honest with yourself, such a hard thing to do! You are at a point that you are getting tired of feeling jealous. Once you get the ability to tame that feeling, you will stop comparing yourself to your presumably prettier sister.
Positive self affirmations, yes, and again, yes! Just as royaltywriter urged you to do. I tell all my self-confidence clients, get in front of a mirror when you utter your affirmations with a smile and with sincerity. Say the affirmations out loud. You will learn to believe them. I didn't invent the method, but I use it just as many other coaches do, because it works! How I know it does? By using it myself when I got advised by my self-confidence gurus years ago.
Keep in mind though, that you will have to make this a habit. You need to persevere in affirming and declaring yourself in a positive light. This means you need to declare yourself daily. It will feel awkward at first. But you will be alone and nobody else will watch. It will take some time because you have hammered that low self-esteem in yourself for years now. But if you are really sick of the jealousy and are highly motivated to allow yourself to live a better life within your own skin, you will make a much faster progress towards self-acceptance and self-love.