Nic Cage's Daughter-In-Law Turns Down 'Relationship Rehab'

Weston Cage and his wife Nikki Williams may have an incredibly screwed up relationship, but they're not willing to spill it on a new reality show.

We're told the fiery couple has turned down an offer to appear on VH1's "Relationship Rehab," a revamped version of "Celebrity Rehab," that deals with bad relationships instead of drugs and booze.

We're told Nikki isn't interested in airing their dirty laundry -- of which there is plenty -- and would rather leave the last four months -- which included two domestic violence incidents that resulted in their arrest (link, link) -- behind her.

Casting for the show is off to a rocky start ... Jonny Fairplay's ex was asked to appear on the show too -- and also turned it down.