Receptive and Expressive Language Delays

I suspect my stbad has intellectual/cognitive delays or deficits that are causing her lack of receptive and expressive language. She was @1mo premature, and has been slow to reach other milestones. She is now 14mo, and still has no expressive language. I had initially suspected a hearing problem, as she did not respond to her name, but I have no longer suspect that is the case (although we are having a hearing evaluation done next month). She is a poor problem solver, and is not responsive to simple commands/requests. I have been trying to teach her sign language for the basics such as "more, milk, all done, eat", but if she does attempt to do them, they are completely out of context. I am not too concerned about autism as she does engage well. Have any of you had experience with a kiddo that just doesnt seem to "get it" (for lack of a better broad spectrum descriptive). I really want to be proactive in getting her the services she needs. I am already working with the birth to three program, but honestly, they have not done any assessments, and haven't been much help. I was wondering if the Bayley Scale assessment, or other assessments should be done. Any insight would be so helpful