Dealing with Family Members Who Deny Your Chronic Illness

Does anyone have any advice on dealing with family members that deny your chronic illness and blame it on your weight? I was diagnosed with Sjögren’s about a month ago. I’m 24 and am overweight. My dad basically has been denying that the diagnosis is genetic and says that if I lose weight I’ll be “cured” and won’t feel any symptoms (I know it’s bs don’t worry) it just hurts to be gaslit after finally getting a validating diagnosis. I have tried to educate him on my autoimmune syndrome but he has just been denying and told me to just get some sun. For context because I am aware this maybe cultural (I am black and Hispanic, my father is black)