Hi M.,
You do have some good suggestions already about talking with your other daughters about letting their younger sister talk on her own instead of talking for her :)
The fact that she is getting frustrated and is not able to get the words out like she wants to could signal that she is truely having diffculties with speech for whatever reason.
You can request a script from your pediatrician to get a speech evaluation- many times you can get this through the county and the eval will be free. I am a speech therapist and I work with ages birth to three. Typically, around age 2, children should be using single words frequently and be putting phrases together such as "more juice", "baby cry", etc. It never hurts to get a speech evaluation. Some kids are just "late talkers"..this is true. However, some kids truely need that extra little "push" and they catch up very quickly. It's better to catch it early than to wait. As a pediatric speech therapist, sometimes when I get a new child who is almost three, I often wish that I had gotten them on my caseload months earlier to help eliminate those frustrations.
Once you get a speech eval, the therapist will determine whether or not your child even qualifies (in Wisconsin, they must be 25% delayed or more- 0-25% is considered within the normal range of development). Then you still have the freedom to decide whether or not you want yoru child to receive services. Typically, in early intervention, a therapist will come to your house/daycare, etc and give you many suggestions and ideas to enhance your child's speech/language development.
If you are really concerned, talk with your doctor about getting a referral and take it from there. Good luck! You will see amazing results :)