How do you know when you've grown up? A member of the '80s Brat Pack with whom you went through adolescence is... is... Heaven help us, she's playing a mom! But Mare Winningham, the plain Jane of St. Elmo's Fire, doesn't seem fazed in the least that she's now too mature for girly roles. Says the female lead of David E. Kelley's fall entry, The Brotherhood of Poland, N.H.: "I've got three sons of my own that are just within two years apart... and I raised them in a small town."

However, the concept of the CBS drama (which debuts Sept. 24 at 10 pm/ET) didn't resonate with the sometime folk singer only as a wife — she plays Randy Quaid's unhappy better half — and a mother. She also related as a sibling. "I'm one of five kids," she says, "and [definitely] there are dinner-table scenes that remind me of my family and the [clan] I grew up in... just how everybody keeps talking louder and louder and louder."

What appealed to Demi Moore and Ally Sheedy's Hollywood classmate most, though, was the secret sisterhood within Brotherhood. As much as the show focuses on the boys, it also will revolve around their spouses — Winningham, Elizabeth McGovern and Ann Cusack (yes, one of those Cusacks). "We married into the [core] family," says Winningham in character, "and it's a lot to take on. So we bond in the pilot over a ladies' lunch, and I think we'll be spending some time at those [get-togethers down the line].

"All three of us [women] are extremely definitive in what we want and expect from our lives and husbands; we don't sit back and take," she concludes. "In my character's case, she has a lot of determination to [fulfill her dreams]. She's like that Jill Clayburgh movie It's My Turn. She thinks it's her turn, big time."