P.O.
If you are concerned about this, you can contact TEIS (Tennessee Early Intervention Services) for a free speech evaluation. These services are available until the age of 3. At the preschool I work at 3 and 4 yr olds have a speech evaluation each fall, but no younger children. I have heard the speech pathologists say that a child should have "one word' utterance at age 1, "two word utterance" at 2, "three word" at 3, etc. If she is not frustrated, having behavior issues or otherwise delayed, it may be more being the youngest child not being a preemie.
My son was born at 28 weeks (26 yr old) and has physical and mental disabilities. So I have experience with the need for pediatric therapy. Child talk at different times. Having older siblings she may be too busy trying to keep up with the older ones. Enjoy her happiness...sounds like she is headed in the right direction.
P.