To address agitation, really listen to your relative as she expresses her frustration. She may give you a clue about what is upsetting her, which can then be used to devise a new strategy or approach. It's also helpful to reassure your loved one that you are there to provide help and comfort.
If it seems like your relative needs something to do, redirecting her to an enjoyable activity -- such as a game or art project that you know she finds pleasurable -- can often improve the behavior. Agitation is also commonly due to a noisy or distracting environment, so relocating your loved one to a calmer area may be all that is needed.
Sources:
Behaviors: What causes dementia-related behavior like aggression, and how to respond. Alzheimer's Association. 2005. http://www.alz.org/national/documents/brochure_behaviors.pdf
Mace, N. L., & Rabins, P. V. (2006). The 36-hour day: A family guide to caring for people with Alzheimer's disease, other dementias, and memory loss in later life (4th ed.). Baltimore, MD: Johns Hopkins University Press.
