How can i help my niece whom i think is losing too much weight?

Many possibilities: Is your niece a teenager? Are you worried about anorexia? Or is she a child? Are you worried about infection, malignancy, or an endocrine problem such as diabetes or insufficient growth hormone? This is a short list of the possible causes. She needs professional evaluation. If she's not a child, see if you can talk with her about seeking medical evaluation. If she is, speak with her mother.