Is it normal for a 2 year old not to be talking yet?
My nephew just says ya ya to everything...My SIL is worried that there could be a problem, such as autism?
First, how old is he exactly? There's a big difference between 24 months and 35 months. Every child is different, and boys are often slower to talk than girls.
Second, does he have any words at all? If he really only says Ya (no Dada, Mama, Goodbye, etc) then I would be concerned. A two year old should be using a full range of consonants and vowels in his babbling.
Third, does he understand what is said to him? Comprehension precedes expression. Does he do what you ask (as in "Give me your sock.") If he doesn't comprehend, I would definitely recommend further evaluation.
Fourth, does he communicate in other ways, for instance by pointing to things, grunting, with facial expressions? That means he's on the way to talking.
Finally, what does his pediatrician say? If his doctor isn't concerned, your SIL probably doesn't need to be. I don't mean that doctors are always right, just that they see a range of kids and know when something's really unusual. (But on the other hand, I would always trust the insistent inner knowing of a mom over a doctor who barely knows the child.)
Even if the doctor is not concerned, it might set your SIL's mind at ease to have a hearing test by a licensed audiologist. Slight hearing loss, which would be hard to identify at home, can definitely interfere with language acquisition. The test is not at all traumatic, is generally covered by insurance or early intervention programs, and since early intervention makes a big difference, it is usually recommended as the first step in evaluating a child for possible language delay. (Getting a referral and then an appointment often takes a month or so.)
If his hearing is normal, and his not talking persists, I would then recommend that he have a speech evaluation.
If the speech evaluation is normal, your SIL should specifically ask her pediatrician about having her son evaluated for autism. Generally, kids with good eye contact who interact lovingly with parents are not autistic, but there can be other reasons for speech delay that need intervention.