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I don't have much more info than what I posted. I do know that part of Posen is now part of Poland. Wilhelmina's husband, Julius Schroeder died in 1889 and she remarried Henrich August Scheffer (Schaeffer) in 1890 in Texas. I have found out that Wilhelmine and her husband, Henrich August went back to Germany in 1913. Following is her obituary that I have. It doesn't have a lot of info, but perhaps something will tie the two together.
Obituary - Banner Banner Press - April 22, 1941.
Mrs. Wilhelmine Scheffer, 81, died at 10:45 Tuesday at the residence of daughter and son-in-law - Mr. & Mrs. Will Heidemann. She had resided in Sandy Hill for 47 years. She was born January 14,1860 in Province Posen, Germany. She was married to Julius Schroeder on November 27, 1880 and they came to America in 1884 and settled in Washington County, TX. Julius died November 28, 1889. She married August Scheffer January 10, 1890. Survivors include E. P. Schroeder of Sandy Hill, Mrs. F.W. Boenker of William Penn, E. C. Schroeder of Sandy Hill, Mrs. Charles Winkelmann of Brenham, Mrs. Otto Ellermann of Sandy Hill, Mrs .B. F. Winkelmann of Robstown, Charles Scheffer of Sandy Hill, August Scheffer Jr. of Austin, and Mrs. Will Heidemann of Sandy Hill. One son died some years ago. Burial will be on Wednesday.
I have researd the LDS website, and have found the there are quite a few Wilhelmine Mantei's, but have not made any direct connection. Do you have the town where your Wilhemine's brothers were born?