When " Real Housewives of Beverly Hills" star Kim Richards checked herself into rehab in December, many people assumed it was the pressure of having cameras follow her every move, and the very public documentation of her fallout with younger sister Kyle on the show that led her to seek help.

But what viewers didn't see on Season One or Season Two of the hit Bravo reality-TV show was the real behind-the-scenes drama that had been brewing for years, both with Kim's battle to stay clean and the sisters' tenuous relationship.

"It wasn't about the Housewives," Kyle, 42, said today on " Good Morning America" of Kim's decision to enter rehab.  "I think it was just an accumulation of things.

"It was our family and sitting down, and she really felt it was time," she said.  "I have to give her credit for making that decision on her own."

Kim's erratic behavior on the Bravo show, from slurring her words and throwing tantrums to moving in with a boyfriend her family did not like, led fans and her fellow Housewives, including Kyle, to believe she had a serious substance abuse problem.

Now, for the first time, the former child actress, 47, has confirmed her addiction.

"I'm an alcoholic," Kim says to Bravo's Andy Cohen in her first TV interview since her rehab stay.  "I'm an alcoholic."

Kim's confession comes during a sit-down with Cohen that will air in the finale of the three-part reunion show that begins Monday.  A sneak peek was made available on People.com, but the full interview will not air until Feb. 13 on Bravo.

"I'm proud to hear her say that," Kyle said of her sister's admission.  "I'm proud of her. I know that's the biggest step in recovery, to admit that."

The strain Kim's substance abuse caused between the two sisters was documented last January at the end of the show's first season in an episode in which a blowout fight between the two saw Kyle scream at Kim, call her an alcoholic, threaten to stop covering for her amid allegations of abuse and threaten to revoke any aid that she and her husband, Mauricio, provided to Kim.

"There were things going on that the audience wasn't aware of," Kyle said on "GMA."  "I had had a lot of pain and built up anger and frustration towards her, and obviously she did towards me too.  It was a vicious cycle.

"It was very difficult to have people dissect and analyze your relationship thinking they know you from a TV show when it's been a long time in the making," she said.

Kyle, who has not yet said whether she will join the show for a third season, said on "GMA" she "absolutely" regrets signing on to the show and allowing cameras to follow her every move.

Kim, who has also not committed to continuing with the show, did not appear with Kyle or the other main "Housewives" co-stars - Taylor Armstrong, Camille Grammer, Adrienne Maloof and Lisa Vanderpump - and newbies, Brandi Glanville and Dana Wilkie - in the reunion because she was in treatment when it was taped the first week of January.

The clips previewed by People show the ladies delving into confrontation and rehashing hot-button issues, such as the suicide of Armstrong's husband, Russell, and feuds, such as the confrontation between Kyle, Kim and Glanville in the infamous "Game Night" episode.

"It's really like mental torture sitting there with all the women," Kyle said.  "We've put our arguments to rest a few months ago and then, all of a sudden, to rehash everything is very painful.  If you're very honest, and people are highly sensitive, it leads to an argument again."

Kim's erratic behavior was a focus of the show's second season, including one episode in which Maloof's husband, plastic surgeon Dr. Paul Nassif, told Richards she was taking too many prescription medications, and another in which Glanville accused Richards of being "wasted out of her f-king mind."

Her strained relationship with Kyle was also front-and-center, as well as a rocky romantic relationship that she felt controlled by and was not happy in, she admitted in the show's second-to-last episode.

"I told her, 'I don't know if you want to watch the show all the time,' because I don't want her to look back and beat herself up.  I really want her just to stay positive and move forward," Kyle said of her advice to her sister post-rehab.

While the limo episode last January led to a months-long standoff between Kyle and Kim in which the sisters had little-to-no contact, the two are working to repair their relationship now, Kyle said.

"I don't think you can love anyone like a sister or fight with anyone like a sister," the "Life Is Not a Reality Show" author said.  "My sisters  - Kathy, Kim and myself - we're all very close."

"We were always proud when one of us achieved something so, right now, for Kathy [Hilton], Kim and me, we're all just very happy and proud of her," she said of Kim.