You SHOULD be healthy. Being attractive is a plus
There is nothing wrong with taking pride in your appearance. The problem comes when people start to think that physical appearance is the only thing that gives them value (I am not saying this is you, I am just speaking generally). It is important to RESPECT your body no matter what your size is, and always important to work towards a healthy lifestyle (for some people that's a size 0 and for others that's a size 14).
How does being healthy and looking great = low self-esteem? I can't wrap my head around that. First of all, being healthy is something we all SHOULD do, but sadly many us aren't taking to time to pay attention to what our bodies are screaming for in terms of need. All I can say is that when you are healthy, trust me, your body is going to repay you. If you take care if it, it will take care of you.
There no limits to Looking Healthy and Great Go The Fullest in your Life and see where It takes you, Low Self Esteem Means low self-Worth, You Want to look The Greatest you can look in this life, Go for it, There are no limits to What You want and can do, Be comfortable for who you are now and move forward in what you desire.
Maybe your sister is the one with low self esteem and doesn't like the idea of you improving yourself. Thus showing her up.
ignore her. she's jealous
There's a difference between wanting to be healthy and wanting to modify how you look so that you fit what society wants you to look like. Which do you belong to?