With 12 years difference between you and your little sister, you are an important adult in her life. My guess is that she has always looked up to you. So, yes, how you behave towards her does affect her. Because you are so much older, you are a role model as well as a sister.
You are asking very, very good questions. At 18, you are at the age where you are sorting through your experiences and deciding what kind of person you want to be. You are trying out being generous and being selfish; being self-critical and self-forgiving; being manipulative and being indulgent. Although you could decide to be narcissistic, you have had the experience of being treated poorly by a narcissist so I don’t think you’ll go that route. But it is a choice. By looking clearly at what you are doing, you are taking an essential step in the right direction.
It’s wonderful that you buy your little sister treats but she doesn’t need those things as much as she needs your loving attention. A few minutes reading to her most nights or coloring with her or just chatting and snuggling will put more into her emotional bank than any candy bar ever could. How you give her your things matters more than the fact that they are old or new. Little sisters love to get jewelry and scarfs and things from big sisters when they feel they are being given something special that connects them to their hero.
Keep it positive. Give her attention and encouragement. You’ll find you get as much as you give from the relationship. It’s pretty wonderful to be seen as special through the eyes of a child. Meanwhile, keep asking those good questions and making good decisions for yourself. Your relationship with your sister is important for the two of you and it’s also important as a rehearsal for how you will be in realationships in general.
I wish you well.