M.R.
Yes, your SIL and you Brother need to seek out a Developmental Pediatrician, not a therapist, and the pediatrician should have made this referal more strongly by now. It is not going to go away. Once he gets to kindergarten next year, they will evaluate him, and she will have few choices about it, a good school district will sue them if they refuse because intervention is that important, and if they can't educate that child without providing special education services, they are not going to just let it go.
I don't like to toss around diagnosis advice, but I have two kids on the autism spectrum, and I work as an educational advocate, this child needed intervention two years ago, and they are not doing him any favors by waiting to have to do something by default. The only thing they have that is free is time, and they cannot go back if they waste it.
Keep encouraging them to seek evaluation. Developmental Pediatricians can be found at every Children's Hospital.
One last thing, what ever they do, they should send this child to school on time. Waiting will only delay appropriate educational intervention for a year, and very often results in full reading failure for children with these kinds of issues.
M.