My mom told me my little sister (14) might be cutting herself.

My older sister and I (20f) were both very calm when we were younger but my younger sister has always been like a little hurricane. She was recently diagnosed with ADHD, is slightly overweight and has a hard time keeping friends.

She used to get jealous of me because I was allowed to stay out longer and do things that make sense for my age because I'm 6 years older, but she has gotten over it most of the time.
She feels left out and annoyed when I hang out with our cousin (19f) even though she is one of my best friends, and sometimes we like to do stuff she is too young to participate in, and sometimes I don't feel like having my 14 year old sister hanging around.

Right now I live in another country. Yesterday, my mom told me that my sister is probably cutting herself. I know that isn't the worst scenario but I still feel like I could do something more to help her. I really don't want her to start drinking or smoking too early, our family has a history of alcoholism and addictive personalities.

How can I be a better sister and a role model she'll want to look up to?

Maybe I'm just ranting and might want to delete this later but it would be nice to get some advice.

TL;DR: My insecure teenage sister is cutting herself and I don't know how to be a better sister.