The pension is regarded as salary.The salary is taxable on the basis of due or receipt which ever happened earlier.Pension received in arrears by your grand mother actually became due to your grandfather when he was alive.Had he been alive, he would have paid tax on the arrears.
Section 159 of the I T Act starts with
159. (1) Where a person dies, his legal representative shall be liable to pay any sum which the deceased would have been liable to pay if he had not died, in the like manner and to the same extent as the deceased.
In the instant case, although the pension became due when your grand father was alive , it was only received after his demise by your grand mother. Therefore , it is taxable in hand of your grand mother who shall be treated as”Legal Representative” u/s 159 of the I t Act. in th year of receipt of arrears.
She will have to file the return if the gross total income of your grand father exceeds exemption in the year in which it has been received. Please remember, relief u/s 89 should be claimed , if it ios beneficial to your case.