Neither Matthew Friedberger nor his singing sister Eleanor - the duo known as the indie art-rock band the Fiery Furnaces, who play the First Unitarian Church on Sunday - has ever been in trouble with the law in the City of Brotherly Love.

Nevertheless, the seven-minute-long, time-signature-shifting first song on the FF's fifth album, Widow City (Thrill Jockey records), is called "The Philadelphia Grand Jury." It finds the fairer Friedberger singing: "Make sure mom don't look at the news/We already know - there ain't no suspense/That the Philadelphia Grand Jury strings me up."

"It's no insult to Philadelphia," says Matthew Friedberger, talking on the phone from his home in Brooklyn earlier this week at the astonishingly early rock-and-roll time of 8:30 a.m. (No, he hadn't been up all night. In fact, he usually gets up at 7 to play piano and write.) "If anything, it's about Philadelphia as a primal place of decision making."

The Friedbergers, who grew up in Oak Park, Ill., are a prolific pair who've released five stylistically varied albums since their debut Gallowbird's Bark in 2003. Each CD has a concept behind it - the songs on 2005's Rehearsing My Choir were all about the siblings' grandmother Olga Sarantos; a forthcoming project will set Giambattista Basile's book of Neapolitan folktales, Lo Cunto de li Cunti, to music.

On Widow City, the Furnaces got hot for the 1970s. "We wanted to use '70s sounds to write songs with," Matthew says. "Not make a pop revivalist record." All the music was played on instruments popular in the Me Decade, such as the Mellotron and Chamberlin keyboards, and the band went so far as to use a vintage mixing board employed by Sly Stone in the '70s. "My sister would know if I cheated," Friedberger says.

To come up with lyrics for songs like "Navy Nurse" and "Wicker Whatnots," Eleanor - who's the subject of "Eleanor, Put Your Boots Back On," by her boyfriend Alex Kapranos of Franz Ferdinand - cut out words from advertisements she found in the back of '70s women's magazines.

"Philadelphia Grand Jury," though, was an exception to the rule. Matthew Friedberger, while reading a book by William C. Austin about 19th-century songwriter Stephen C. Foster, found that Foster had written a song of that title. He took it, and wrote his own song.

"The idea in my head as I was writing and recording it is that the character in the song is expecting something bad to happen," Friedberger says. "The grand jury is like a set of fates, or gods, that are going to make the decision about your life anyway, no matter what you do.

"Those are the kind of things you think about when writing a song," the songwriter says. "I don't expect people to get all that when they hear it. But those kind of things animate it for you, and allow you to put enough atmosphere into a song so that people want to hear it."

Fiery Furnaces with MGMT at 7:30 p.m. Sunday at the First Unitarian Church, 2125 Chesnut St. Tickets: $12. Phone: 1-866-468-7619, www.ticketweb.com, www.r5productions.com
Contact music critic Dan DeLuca at 215-854-5628 or ddeluca@phillynews.com.