My cousin walk after 2.5 year,

Does your cousin have any help, such as speech therapy and/or physical therapy?  If not, she should be evaluated and it seems like she would more than qualify for these services.

It's really hard to say what this is without knowing so much more. Even in children with autism, 2.5 years is late for walking. Was she crawling and getting around before that? There could be a motor/movement problem...something like Cerebral Palsy...that would explain the significant delay in motor (muscle) skills but maybe your cousin is just as bright as her peers and is using language such as gestures to communicate?

Does her family seem concerned and able to help her and get her services to get her walking, talking, etc.? They may already know what's wrong and didn't want to trouble you. You can ask her parents if there's a special way you can play with her to help. You seem concerned and I'm sure you love her.  I wish her well!

My son was exactly the same way. He was diagnosed with ADHD. Didn't learn to speak until almost three. No eye contact either until very late, criesd all the time unless he was taking sonrething apart. He is very good at building anything.. He loves to take things apart and learn how to put  them together as a child. His childhood was a constant battle....never a moments peace always into something and trouble at school. I know now that he is an Aspie  with. ADHD. in the 70's there was no Aspie diagnosis in the autism spectrum . So we tried counseling, diet, meds, special schools everything. 

He is now 40 years old and still on meds for various disorders as well as ADHD. He is a remodel contractor. 

My other sons Aspie spectrum a computer prodegy  had twins and one twin is diagnosed in the Aspie spectrum & he takes anxiety drugs as does my son which helps him focus and calm him.  Anxiety is the driving force as they get older. This runs in our family. I wish I knew this earlier but had no way of knowing. Now both my sons have passed the gene and now their children have to decide to have children.  

However on grandson is severe Aspergers and will probably never be able to function on his own. Very heartbreaking. 

I HOPE that with time an appropriate diagnosis for your cousin will help him or her find his own path in life as he grows. 

Kind Regards

Hope

Developmental delay, especially delay in the acquisition of the motor stages such as walking can be initial sign of many disorders or even can occur in healthy children.

About speaking, it is important to note that children with autism often learn some words, but about the two years of old they can not say words that before learned, in other words, they are backsliding!

However, I recommend visit a pediatrician, psychiatrist or occupational therapist for comprehensive and accurate assessment.