Your description of the situation makes me suspect that your mothers tuner is being overloaded by the strong local signal.
In some cases, the situation can be managed by inserting between 10 and 60 dB of attenuation in the line between the antenna and tuner.
http://www.3starinc.com/drop_in-line_attenuator.html Weaker signals may be made too weak to receive so some experimentation will be required to determine the ideal amount of attenuation in each case.
In other cases, a highly directional antenna may be able to 'ignore' some of the strong local signal while increasing the amount of signal received from more distant stations.
I would strongly advise against using any sort of amplified antenna or accessory amplifier.