My sister passed away 40 years ago and was buried in a wooden coffin. My brother wants to exhume her remains to be cremated. I need to know how long does it take for a 3 month baby body to decompose and will there be any remains left to cremate?

I have to ask why? After 40 years. A lot depends on where this is and the local soild conditions... In an arid climate like the high desert of Nevada... a long LONG time... in a place with a high water table and a lot of rainfal like Florida or Louisianna... not very long at all.

She was buried in Mexico. The family resides in the US now. It's been over 6yrs since we gone to Mexico due to the bad violence at the border. My brother did some research on it and mentioned it to us. But realistically, would there even be anything after 40yrs? Not to mention, she was a 3mth baby.

I called my cousin who owns a funeral home and went to mortuary school. He said it is extremely unlikely there would be any body to cremate after 40 years. If anything, bring the unopened coffin to this country and rebury it or cremate the unopened coffin.

And a wooden coffin, not in any other protection, it would not be in one piece either ( depending on conditions)

This is one, where a nice memorial perhaps in memory and allow her body to rest in peace. And of course while there is a lot of violence, there is also 1000s of people who travel back and forth ever day

The documents you will need and permission to cross the border will probably be daunting. It is understandable that officials will be suspicious at every step.

Try to think of her as part of the earth where she is... and possibly better there.
Make a little shrine for her here - in your yard, your house, plant a little tree or shrub... just some ideas for comfort. Let her be.

My funeral home cousin also mentioned what joypulv mentioned--getting the coffin through customs might be a ton of paperwork and nearly impossible. Yes, let her stay where she is and plant a tree or pretty bush or little garden in her memory.