Maybe you have discussed this already, there are cases in which the birth certificate has not been registered on or around the date of birth, but months or years afterwards. It is my understanding the birth certificates are filed by the date of registration, not by date of birth. Also, another factor is where was the birth registered.
I only bring this subject up because last year my wife and I were chasing down a birth cerficate for her sister. Her sister was born near La Romana and my wife and I spent days going back and forth from one govt office to another there looking for a birth certificate and even to a local church to inquire about baptismal records.
Well, we found the baptismal record in Higuey dated two years after her birth and we used the information, her father declaring the birth two years later, to go back to La Romana to search the registry books, having to find out we had to go to the Capital to Oficial?a de Estado Civil because the books were there being reconstructed. End result. we got the birth cerificate after some more hassles.
So, if you have not already done so, you may want to ask your mother-in-law and or her family members when and where did her father declare her.
Regards, PJT