'Sister Wives' Kody Brown Divorces One Wife Marries Another

2/3/2015 12:30 AM PST

'Sister Wives' -- Kody Brown Divorces One Wife, Marries Another

TLC's "Sister Wives" star Kody Brown has traded one wife for another ... TMZ has learned.

Polygamy is confusing so here's the deal. Kody claims to have 4 wives, but he's only legally allowed to marry one. He married Meri before meeting any of the other women, but we've learned he decided he no longer wanted her as the official wife.

So Kody quietly divorced Meri and then tied the knot with another sister wife -- Robyn, the youngest of the bunch.

Now you wanna hear a friendly divorce. Meri is still in the fold, and she's become one of the backup singers ... aka sister wives.

As for why Kody made the switch ... we're told he wants to make sure Robyn's kids have security within the family, such as access to insurance and other benefits.