A couple of thoughts off the top of my head:
1) When you feel strong enough, I would tell them with as little emotion as possible that you don't like it when they use your babysitting services, yet don't treat you as lovingly as other family members...for example not sending you a birthday card.
2) If there is a relative that you TRUST, I would ask them quietly and calmly if there is something that you might do that would improve upon the situation. I would keep it short and brief. Then I would consider what was said. Do not take anything emotionally. Just consider it. Make changes if they are not offensive to you.
3) I would make sure to develop my own interests, if you are not currently doing this. Explore hobbies and interests. Make sure you are happy and satisfied in all areas of your life. At some point your family should become aware of this. If they don't become aware, I would subtly make sure they do figure it out.
4) If the situation does not improve, even after you have calmly made sincere efforts, I would stop babysitting.
5) Ditto to what Mindseek has said regarding about what we think and say about ourselves.
Wishing you well.