Pregnant Robyn Brown called her polygamist husband Kody "a hero" on Father's Day. Credit: Bryant Livingston/TLC

Seventeen kids and counting! Just two days after announcing the Sister Wives family will be expecting their 18th child, Kody Brown's pregnant wife Robyn lavished praise on the controversial reality TV dad. "Happy Father's Day! You will always be a hero in my book because of your love and devotion to all of our 18 children," Robyn, the youngest of polygamist Kody's four wives, tweeted on Sunday, June 21.

An official member of the Brown family since 2010, Robyn, 36, joins Kody's other sister wives Meri, Christine, and Janelle. Earlier this year, Kody and his spouses announced that they were "restructuring" their family, and that Meri was divorcing Kody (of her own volition) after 24 years, so that he could legally marry Robyn.

"We have chosen to live in a plural family for many reasons, above all we have a testimony that this is what God wants us to do, and that it will make us better people," Kody has said. "We value family above all else."

To thank their husband for his devotion to their family, Christine and Janelle also posted sweet Father's Day Twitter messages, calling their kids "blessed" to have him as a dad.

Kody's 18th child will be the fifth for Robyn. In 2011, she gave birth to Kody's son Solomon, and has three children from a previous relationship.

Sign up now for the Us Weekly newsletter to get breaking celebrity news, hot pics, and more delivered straight to your inbox!