I'm sorry that people are adding so much stress to your life. First of all, while some kids develop faster, and others slower, by kindergarten most kids are on level playing ground. Your mother shouldn't even be considering Autism. Autism is characterized by normal development and then around age 2 there is a regression, most notably in language skills. Tell her to "chillax" as the speech kiddos I service would say :)
Secondly, I'm not sure what things you are think are wrong with Brayden, but I would recommend not over analyzing everything. Remember that she was pre-mature which already makes her hill a little steeper to climb, and like I said before, in the end, the odds are in her favor that by the age of five, no one will ever know she was pre-mature.
If you are concerned, have evaluations done through AzEIP, Arizona Early Intervention Program. If she does require services, they are free until the age of three, and come to your house!
Kids develop in their own time, and I know that more now that I have two little girls. Brooklyn is so different than Madison was at her age. Madison was the slower of the the two with things like holding up her head, holding her bottle, being interested in her toys, etc, and now, she is a healthy, happy, and talkative 2 year old who is now ahead of the game. She's talking in sentences! Anyway, my point is, relax a little and enjoy every moment you have with your precious little girl. They grow up too quickly :)
Oh yeah, and remember, to your husband, he may see that she is perfect in his eyes, regardless of what delays she faces or if there is ever anything wrong with her.