FLAGSTAFF, ARIZONA: Co-stars Robyn and Meri Brown will share the secret of the decline of Kody Brown's polygamous marriage in the upcoming episodes of 'Sister Wives' Season 18.

In a preview clip released by TLC, Meri tells Robyn, "I have a secret. It was before you came into the picture."

She explained, "Kody and I, our relationship has always been, (gesturing ups and downs) wohooo, like a really fun roller coaster."

She claimed there was a time in their relationship when things were not so good between them and Kody kept telling her, "You're getting all this extra time. You're like 'I'm with you all the time'. I'm with you more than Janelle or Christine."

After listening to all these claims, Robyn appeared to say, "I don't see it."

She then started keeping track of how much time Kody spent with each of the sister wives. And, to her astonishment, she ranked less than the third.

Robyn told Meri, "That's really funny that you have that secret because I have the same one."

She explained that Kody spent almost a month with Christine, and she didn't get as much time with him as the other sister wives.

'Sister Wives' star Christine Brown shuts down Robyn's claims

Christine admitted that there was a month when Kody spent around 25 days with her. But, that happened only for a month.

The star claimed, "And then it went right back to where we really didn't see him that often. It was only in the evenings."

Christine added, "He (Kody) just preferred to be over Robyn's house the rest of the month." She claimed, "For 1 month I got him as much as Robyn got him the rest of the time, the rest of the 59 months because he was not at Janelle's house either and we all know he wasn't at Mary's house."

Kody Brown calls wives' comparisons 'death of joy'

Kody became visibly angered seeing his sister wives comparing the time he gave to each of them.

The star claimed, "I think there is a tendency for wives to not see what I'm doing for them."

He added, "But very much to see what I'm doing for someone else." He then claimed, "There is this comparison thing, and it's just the death of joy."

It appears that Kody's marriage with three sister wives failed solely due to the comparison factor.

'Sister Wives' star Christine Brown to snub Kody Brown as she celebrates her 50th birthday

In the upcoming episode, we will see Christine and her daughters preparing to celebrate her 50th birthday. But, Christine will not be seen inviting anybody from the Brown family to her party.

She will not send the invite to Kody and Robyn. However, the star is in dilemma whether to invite Kody's mother to the party or not. She will eventually decide not to invite her.

Christine acknowledges that there are some members of Kody's family who are very close to her and she would love to invite them to the party.

Let us wait to see whom Christine invites and whom she doesn't.

'Sister Wives' Season 18 releases new episodes every Sunday at 10 pm ET on TLC.