My GGG grandfather was Mathias Yerger (born December 25, 1833). On his death certificate, it states that his parents were Mathias Yerger and Mary Bossert.
The 1850 PA census, Montgomery County, Whitepain township (Whitpain), page 251 also supports this information. It states that the elder Mathias was 55 years old and was a farmer. Mary was 54. The children left in the house at the time of the census were: Susanna (18), Mathias (16), Thomas (14), and Charles (12).
I am wondering then if the Susana that is Mathias's sister could be the Susanna you are looking for?