Rosetta is correct. Incapacity Benefit itself is not means tested, but is dependent on NI contribution history and the level. Perhaps another benefit your sister was claiming on top of Incapacity Benefit was lost because of her pension so it looked as if her IB had been reduced, Mrs Overall.
http://www.askcab.co.uk/money_site/mINCA.asp.
My friend has just been told she cannot claim IB as she hasn't paid sufficient contributions in the relevant qualifying period.
However, the new rules will come into effect come October, so perhaps everything will change.