Hewitts of Saleen

Hello, I am trying to find information about my 3rd great grandmother Jane Hewitt. She was born 18 Oct 1831 in Saleen, Cork, Ireland. In the 1850 US Census of Dimmick, LaSalle, Illinois she was with her brother and sister living with a William Longworth and his mom. Jane Hewitt 18, Catherine Hewitt 17, George T Hewitt 15. It says they were all born in Ireland so they must have come to the US between 1835 and 1850. I can find no sign of their parents and do not see them on any passenger lists. Maybe the parents died before coming to America?Jane and Catherine went to Oregon on the Oregon Trail. George ended up gold mining in California. Any information would be appreciated. Brandi