Autism should be diagnosed by a pediatrician, but they will factor in a parent's observations. In the case of a non-infant "child", I would a pediatrician should be able to diagnose directly from the child's behavior.

Regular therapy is good for anyone, autistic or not. So, no big deal there. However, many autistic people are subjected to ABA (Applied Behavior Analysis), which is gay conversion therapy rebranded, and which causes trauma, PTSD, burnout, and suicide. The way to actually handle having an autistic family member is to respect their needs and honor their choices, so that they can self-regulate and avoid overstimulation and burnout. If your father does that for your sister, and she isn't autistic, she still benefits. (Honestly, all people benefit from being treated with the gentleness and consideration that autistic people need.)

Reassurance goes a long way with a child, autistic or not.

This makes no sense. If she's autistic she's obviously been that way for years. You don't just all of a sudden become autistic. Wasnt this looked into when she was little?