Dear Beth,
I agree that the most pressing need is to have your sister undergo and evaluation by a team of developmental specialists. Without such an evaluation it is impossible to determine the nature of the symptoms you describe. They may be manifestations of a developmental disorder, and it is important to figure this out ASAP.
A decent developmental screening can be done by your local school district or health department. Before the age of 3, the program that could be helpful is called Early Intervention. The evaluation is free, and the services or therapy they determine to be necessary are also free. If you disagree with their results then you can ask your sister's pediatrician for a referral to a developmental specialist. Often though, the EI evaluation is enough.
Speech and Language therapy in particular are most effective when begun before age 3, and continued from there.
Developmental disabilities can be overcome, but they need to be addressed as early as possible.
Good luck, A.