"... In the database of SSDI I found her name: Laura Sellin (born 10. April 1887, died May 1986). Who can give me some more information about Laura Sellin, her children, her husband or her mother?"
This is an old posting, but you should consider ordering her death record from Milwaukee County, or the State of Wisconsin. These records are not necessarily available to browse by a volunteer without either advance appointments or fees.
Search for the Milwaukee County and Wisconsin State government sites (.gov), and not the other bogus records sites.