Chloe Sevigny stars as Nicolette "Nicki" Grant on HBO's Big Love. Nicki is the daughter of Roman Grant, the prophet of the compound of polygamous Mormons in which Bill was raised.
When Barb was sick with cancer, Bill brought Nicki in to help raise the kids and eventually married her as his second wife. Bill then had two children with Nickki, Wayne and Raymond.
Nicki is definitely a source of storylines having had problems as a compulsive chopper, a gambling problem, she took birth control behind everyones' backs, and took on a job she lied about and eventually kissed her boss.