Once an SSN is marked as used or deceased by the IRS, that's it. The IRS locks that SSN permanently and forever so that it can't be used fraudulently. You have absolutely no choice but to mail the return to the IRS at this point.
The reject code IND-901 indicates that the social security administration has been notified the person is deceased. The IRS pulls all of their information from the social security administration every year. The only way possible to fix this, is for the owner of that SSN to personally visit a local social security administration office and prove they are who they say they are so that they will fix the problem. The bottom line is, you have to fix this with the Social Security Administration. You can "NOT" fix it any other way. Additionally, you can "NOT" fix it so that you can e-file the 2019 tax return. But you can fix it so you can file 2020 and later tax returns.
Understand that this not only affects taxes. It will also have a direct impact on your MIL to collect *ANY AND ALL* social security benefits that she may qualify for in the future. So the owner of that SSN must fix this in person themselves directly with the Social Security Administration. Nobody else can do it for them.
