Cow's milk and a nine month old???

i am a pediatric nurse practitioner by day. I have three of my own kids, from 11 months to 6 years. I see patients all day long. I have a sister that has a 9 month old. She has nursed him up to this point, but now she is giving him cow's milk in a bottle. I have TRIED to tell her the dangers of this, even given her reading material about it. She called my office and asked me today how she could get him to stop nursing at night as well and just get him on the cow milk all together. I told her that I thought it was a horrible idea to do that and she then doubted my medical knowledge as her pediatrician ( full fledged Dr., not just a "Nurse Practitioner" like me...) told her it was okay to do it. What can I do! I need to let her know that she is making a bad choice. Maybe if you all answer this question and she can see the vast majorities opinion, it will help her understand. Thanks!