Immigration from Norway - Johnson


I would love suggestions in tracking down my 2nd great grandmother Syverine (Susan) Johannesdatter (Johnson) and her sister, Lina, who emigrated in the late 1880s (I suspect) from Naes, Norway.   I know that they had to be here by 1888 when Syverine when she married Karl Kristianssen.  

Syverine was born 10/17/1856 and Lina was born 8/210/1864.  Both settled in Stoughton and Madison, Wisconsin

Any suggestion would be great.