All Indian residents having the age of 50 years or more fall in the Non-ECR category (Source). As your mother-in-law is aged over 50, she will not be given an ECR stamp in her passport.
The revised list of documents which can be used as proof of date of birth mentions the following:
- Birth Certificate (BC) issued by the Registrar of Births & Deaths or the Municipal Corporation or any other prescribed
authority whosoever has been empowered under the Registration of Birth
& Deaths Act, 1969 to register the birth of a child born in India
- Transfer/School leaving/Matriculation Certificate issued by the school last attended/recognized educational board containing the DOB
of the applicant
- PAN Card issued by the Income Tax Department with the DOB of applicant
- Aadhaar Card/E-Aadhaar having the DOB of applicant
- Copy of the extract of the service record of the applicant (only in respect of Government servants) or the Pay Pension Order (in
respect of retired Government Servants), duly attested/certified
by the officer/in-charge of the Administration of the concerned
Ministry/Department of the applicant, having his DOB
- Driving license issued by the Transport Department of concerned State Government, having the DOB of applicant
- Election Photo Identity Card (EPIC) issued by the Election Commission of India having the DOB of applicant
If her Aadhar card and voter card have her full DOB mentioned, then those can be used.
NOTE: The Document advisor does not list all documents acceptable for age related Non-ECR category but I think the latest MEA advisory is the one applicable here.