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If social security is the only income that your parent receives and his/her filing status is single, then it is not considered taxable income. If your parent receives other income but the sum of 50% of the social security benefits plus all other income is less than $25,000, then none of the social security is taxable.
I am sorry that I did not respond sooner but I was away from my PC. If you provide more than half of her support then you can claim her because her social security is not counted for the $3,400 test because it is tax-exempt in this situation.
See page 14 - http://www.irs.gov/pub/irs-pdf/p501.pdf