Brothers & Sisters Brothers & Sisters

If you thought the Walkers had issues on Brothers & Sisters, wait until you see the Laurents in action.

"They may put the Walkers to shame," Gilles Marini, who plays Luc Laurent, tells "It is just pure drama, through and through. There's no other way to describe it. I don't think the Walkers have even dealt with something or someone like that before."

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That "someone" is Luc's mother, Gabriela (Sonia Braga

), a life-of-the-party former screen siren who unexpectedly drops by Sunday's episode to visit her son and his new fiancée Sarah (Rachel Griffiths). Except she's not exactly welcome, as mother and son have been estranged for years.They do, however, pick up right where they left off: yelling at each other in French. "He cannot be in the same room with his mom just because of what she's done in her life to him," Marini says. "What she did was bad. He thinks she's selfish. Luc just does not accept the way she is."You've never seen Luc upset," he continues. "Upset is a small word for what's going to happen. He's crushed and destroyed from the inside out."What did Gabriela do that was such a devastating blow to him? It stems from his childhood, Marini says, which is more than Luc can share when Sarah tries to broach the subject with him.

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"You learn his issues that are very deep," the actor says. "There are two sides of the story. When you're a kid growing up, you only see your side. The only thing he kept in his heart is his childhood. When he sees her again, she [awakens] the bad memories."The truth about the Laurents' rift will be revealed by the episode's end in what Marini describes as "the worst possible way." That's all thanks to Gabriela's meddling in the Walkers' lives: The feisty free spirit becomes the temporary third wheel in the budding relationship between Nora (Sally Field) and Dr. Karl (John Terry) and, as the promos tease, she's going to lock lips with Justin (Dave Annable). "Poor Nora! She's in the middle of the havoc," Marini says. "Unfortunately everything comes out as a big bang. This is why I keep saying she's like a nuclear bomb unleashed on the Walkers. They think they're dysfunctional and now they're like, 'Oh, God!'"

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Braga was only attached for one episode, but the effect of Gabriela's visit will linger for the rest of the season, as she forces Luc and Sarah to be more open with each other. But don't be alarmed when Luc is MIA for three weeks afterward — the French stud is overseas for work. When he returns, it may be time to start planning a wedding."I believe Luc and Sarah belong together. I hope with all my heart that's where it's headed," Marini says. "They've had a very romanticized relationship at first, but now, things get more real and grounded. I like that there are darker moments like this. They're real people. Can they work it out and get married? I think so."And will Laurent mère be invited to — or crash — the wedding?"I hope so," Marini says. "Sonia was amazing. Gabriela was a breath of fresh air to the show. Maybe she'll kiss someone else at the wedding!"