ET was exclusively behind the scenes when "Brothers & Sisters" filmed its season finale -- and now we can show you the history-making television event before it airs on Sunday night.

On last week's episode, Kevin (MATTHEW RHYS) proposed to his same-sex lover, Scotty (LUKE MacFARLANE). This week, the two will celebrate their decision to be life partners with a commitment ceremony officiated over by Kevin's sister Kitty (CALISTA FLOCKHART).

"It just feels like the natural flow of the Walker family," ROB LOWE, who plays Kevin's brother-in-law Senator Robert McCallister, tells ET's JANN CARL, who is on the set for the event. "It doesn't feel gratuitous, or really just standout. That's where Kevin was heading and it's really organic which is really nice.

"Ten years ago, there wouldn't have been a gay character portrayed the way the show does in primetime, I don't think. I also don't think we would be able to do the same kind of political storylines that I get to do on the show."

Rob's character is a Republican senator who voted against gay marriage, even though he has a gay brother. And McCallister's brother, who is a minister, previously dated Kevin. All that aside, when Kevin realized he forgot to get a ring, the ceremony comes to a halt, until McCallister loans them his.

"There's a lot of interesting stuff that has been coming up for me, which is good," Rob continues. "That's what you want as an actor. You want a lot of paint on your palette to paint with."

"Brothers & Sisters" airs Sunday night at 10 p.m. on ABC.