So 1934 is not early enough your mother-in-law has to ask at the registry office for a birthcertificate of her father. Thats a bit difficult, cause the Polish office needs Polish text.
Here is the adress:
Urz?d Stanu Cywilnego Kierownik: Miros?awa Winnicka tel: 095 7355 827 fax: 095 7355 827 e-mail: email@example.com adres: Urz?d Miasta Gorzowa Wlkp., ul.Kazimierza Wielkiego 1, 66-400 Gorzów Wlkp. pok.: 1
It has an emailaddress so she can ask about the certificate. But try to find someone who translate the questions to Polish. The answer can need some weeks and will also be in Polish. I think she has to prove, that she is daughter of her father, so it will be advantageous to sent a copy of her own birthcertificate. It will cost something.
And I'm sure that the name of her Grandma would be "Krüger" with two dots upon the u, cause this will be a general German surname. I'm also sure that the correct name is: Conrad Sauck.
If she got the original birthcertificate of her father she will see if there was a father or not. Often women got childrens without beeing married. Sometimes they married the real father later. Than it is noticed that the father accepted his own natural child.
If she got the certificate and found nothing about the father, she can ask at which church the christning was. Than try to get the christning certificate. Pastors sometimes make a note about the father.