Tax Credits and DLA

My sister claimed DLA for her disabled son and was awarded low rate mobility.This entitled her to higher rate tax credits.However shortly after the mobility was disallowed. She informed IR and tax credit was cut and she repaid money overpaid. However she appealled decision to disallow DLA and after a hard battle lasting over a year Tribunal ruled in her fovour and reinstated DLA back to low Mob and backdated it to original claim.I thought that she should also get tax credits back to original date of claim however she has been told by IR that they will only pay her back 3 months.
Can anyone help?