My step-daughter is doing marvelous teaching/playing exercises with her daughter (18 months). She is doing many of the things Will recommended. Also consider asking the parents if they have/approve of the Baby Einstein CD's. There is another addition for young children.
Discuss with the parents your intention to encourage interaction between the two siblings via various modalities. (Sorry, just read the last page editorial regarding overachieving parenting from my Time subscription.)
Taking that "various modalities" psychobabble seriously, consider this possibility that my step-daughter is using. She has been teaching her baby ASL (American Sign Language) and there is a baby version. I think you could engage both children in the "what is this" game that also allows them to "communicate" with each other.
Eh, no parent here, just a totally amazed grandma.
Your enthusiasm and interest in providing a quality play/learning experience may be all that you need. Your current education level, and the difficult role you are providing to a difficult group of teens may also have positives.
Best to you, Ms. Snowy. You have the qualifications the parents are looking for.
