My cousin 12 y/o with mild autism. what should i do, if no one is forcing her to eat she could not eat all day? now she is underweight.

Mothers helper: Try to involve her in the preparation of meals it would be a new activity for her. While helping you, you might ask her to sample the food for taste and texture. She might enjoy the new activity and that way you might get some calories in her.

Coercion creates: Oppositionality, esp. In kids with autism, all of whom have deficits in theory of mind, the ability to understand that others can have a different point of view or emotional reaction than one's own. Therapy must be repetitive, intensive, " rules-based" & presented visually; e.g., "time to eat when the timed-timer hand is on blue." if her bmi is <5th%ile, rule out genetic & other medical causes.

Consultation: I would start with her pediatrician for suggestions. In addition, I would consult a behavioral psychologist who has experience with Autistic Adolescents and with body dysmorphic disorder.