Filing of Income Tax Return of a deceased man by his surviving second wife

I, a widow, had married a widower. I had no child from my first marriage. My second husband had a son and a daughter from his first marriage, who did not live with us. My second husband died at the age of 73 years, in December, 2017. All of our bank accounts and investments were in joint names, with either of us as the prime account holder and Immovable properties are in my name only. So, I had no difficulty in making the joint bank accounts as single (in my name) and receiving the proceeds of all other investments (mostly bank F.D.s or Post Office schemes) after submitting the death certificate of my husband in banks and Post office. None of his children were nominees in these accounts or investments. I am a pensioner and my husband too was a pensioner. Disbursement of Family pension is yet to be begin. I have been differently advised on the question - whether to file the IT Return of my husband, whose annual pension was for 2017-18 was around Rs. 2.7 Lakhs (for 9 months of his survival in 2017-18) and TDS was effected on investments. One advice is in negative, suggesting that the IT liability of a deceased is not absolute and his tax liabilities are gone with the man. Another advice is that whether a tax liability or not, whether a refund or not, the IT Return must be filed mandatorily by his legal heir. My question is - whether it is mandatory for me to file his IT Return. If Yes, whether I have to first obtain a Legal Heir Certificate. If Yes again, how to get the Legal Heir Certificate and whether my deceased husband's son and daughter have to be involved, as the process would be a non-starter in that case since they would not cooperate.