Though in a season 3 episode, it's stated that Eric was an accidental child.
Kelso attempted to end what he had with Laurie multiple times, only to be threatened that if he ended it, she would tell Jackie everything.
She also showed genuine sympathy towards her brother when he was traumatized after seeing their parents having sex by holding him close to her and telling him it'll be okay.
After season six, Laurie was never seen on-screen again but was mentioned in passing many times over the next two seasons.
Crushed, Kelso ended the relationship.
During the fifth season, Laurie returned for Thanksgiving and was mentioned to be now living in Chicago and after accidentally letting it slip she hadn't changed her promiscuous ways, she lost her spot as the favorite child.