Here's another suggestion - try looking for church records where the sisters lived. If they were in a German-speaking community, chances are that the church records might have more detailed information. For example, when I was researching my husband's family in Illinois, the church baptism register listed the place of origin for the parents, not the place where they were currently living. Sometimes there is also a membership list, and that might contain clues as well, especially if you can find the year they joined the church - then it might say where they came from (usually it would be another church, but in this case, might list the home town).
Re the first names, Gale is absolutely right. Usually girls were named Maria Something or Anna Something. Since everyone had Maria or Anna as a first name, they were more likely to be known by their middle names.
Another thing you might try is this website -
You type in the surname you're looking for, and it will show you where that name appears today in Germany. The darker the color, the more people there are with that surname. Try all the variations that Gale gave you to see if that helps narrow down the search area.
To get more variations on surname spelling - because spellings changed over time - try here -
Type Strubic or variation in the surname field and see what else comes up.
Hope this helps.