How do I tell her I think he is autistic?

Hey everyone, I need some help. My SIL’s oldest is 3. He was the perfect infant ( never cried, calm, content) as soon as he turned 1 it all changed. He is the most unhappy child I’ve ever met. For the last two years it is non stop tantrums and meltdowns, aggressive behavior, defiant behavior and screaming for no reason. It’s gotten so bad that his daycare is very concerned. He doesn’t seem to know how to interact with others, hates being held or snuggled and doesn’t listen. It’s to the point that my kids and other fiends kids don’t want him around and refuse to play with him. I’m at a loss and I want to help her so bad but I don’t want to upset her. Have you ever had to bring this up to a loved one? I think he needs to be evaluated and I think he needs so type of therapy to help with his behavior. This is not normal toddler behavior which is what she keeps saying and that he will grow out of it. I think she is in denial and the last thing I want to do is upset her, but I love my nephew so much and it’s breaking my heart to see him struggling so badly.