EXCLUSIVE! 'The Big Bang Theory': Penny's Mother and Brother Have Arrived -- Watch the Awkward Family Reunion!

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It's the Big Bang Theory moment we've all been waiting for!

After nearly ten years of vague stories and M.I.A family members, we’re finally going to meet Penny's mother and brother in the season 10 premiere -- and only ET has your exclusive first look!

In our sneak peek above, Kaley Cuoco reunites with her 8 Simple Rules co-star, Katey Sagal, who is once again playing the role of her on-screen mother. Sagal and Cuoco starred as mom and daughter for more than 75 episodes in the ABC comedy from 2002-05.

30 Rock
's Jack McBrayer also makes his Big Bang debut as Randall, Penny's brother who has infamously been mentioned on the series as a drug-making criminal.

MORE: 'Big Bang Theory' Welcomes Katey Sagal and Jack McBrayer as Penny's Family Members

The Big Bang Theory
's season 10 premiere, which airs Monday Sept. 19 on CBS, is bringing everyone's families together to celebrate Penny and Leonard's wedding ceremony. (Fingers crossed this means we're finally going to learn Penny's last name!)

In the clip above, Penny and her dad, played once again by Keith Carradine, are sharing a beer and chatting about her success as a pharmaceutical sales rep, while anxiety-ridden Susan (Sagal) chastises Randall for not living up to his potential.

"Hey! She just sell drugs, I had to make 'em," Randall proudly quipped.

Back in July, McBrayer made a surprise appearance at The Big Bang Theory panel at Comic-Con and joked that, unlike his character, he’s only been to jail “recreationally,” but he looks "real good in orange."

Season 10 of The Big Bang Theory premieres Monday, Sept. 19 at 8 p.m. ET/PT on CBS.

Dying for more Big Bang goodies? Press play on our exclusive clip below to find out how Kaley Cuoco defines "love."