Should I approach my sister-in-law about her son?

My nephew is three years old and does not talk other than the words daddy, car and baby. He throws fits when he can't get his point across. He does do sign language at times but usually gets frustrated with that too. A few people, one a nurse and another a speech therapist told me they think he may be autistic. And I was told by my mother-in-law that she thinks he should be checked out. I researched autism and speech development in general just to make sure that they weren't just being dramatic and I sadly found they did have reason to be alarmed. My sister-in-law and I are very close and when I asked her about him being a late talker when he turned two, she said she would take him to see someone if he's not talking by five. Being a gradeshool teacher, I know well that children should be talking by then. So my question is, is it normal for a three year to not be talking and if not how should I approach her about it. I know no one else will. And I know if my mother-in-law talks to her, she will assume she is calling her a bad mother. I really just want to help.