The Becker's Email
If you can find your gggm and her sister on a ship's manifest it may give the name of the person they were going to in the US and confirm they were going to Mobile.
How was the Schur surname for your gggm and her sister spelled in early Mobile records as that may be a clue to the brother's surname.
"Early 1900s"--check census records to find out where these two women were living. Did they live with their brother at first? Did they marry in Mobile. In general, researching them may provide clues. Also, such research may confirm where they were from.