Mom in law passed away in 2018. How do we pay her taxes?

MIL passed away in December 2018. My husband, her son, was her POA/Guardian. She was in a care facility here in Oklahoma. They/the state took everything she had, i.e., her social security, her pension, made us sell her house and car, turn over all funds to them. She had nothing left, there is nothing left.

We had been having her taxes done up until 2016. When we went to have them done, we were told by the tax preparer that we had to have a state issued ID - her drivers license had long expired. She was Alzheimers/dementia, so there was no way of getting her out of the facility to have this done. She was incapable of moving any longer. We were told that we could not get her taxes done, and were told to not worry about doing it since she was in the nursing home.

So now, since she has passed away, we need to do her final taxes. We had not received any 1099R from her retirement or the SS statement that were sent every year before. I called her Retirement office, and was told that the 2017 1099/SS had been returned to them, but had been sent to the care facility! 2018 was not returned to them, but we do not know where it ended up. We have mail carriers that aren't too smart sometimes, so there is no telling where the forms went. We have requested copies to be sent to us.

Can we file back taxes for those years that we didn't? We haven't been contacted by the IRS saying that she was in arrears for anything. I'm not trying to stir the pot up, but we need to do know if we should still file for 2016, 17, and 18. Can someone please advise as to what we should do, if anything? Thank you for your assistance!