ABC’s once-unbeatable Sunday night was bound to suffer when the network moved “Grey’s Anatomy” to prop up Thursday. But with “Desperate Housewives” falling and “Grey’s” replacement, “Brothers & Sisters,” continuing to lose audience week after week, ABC’s Sunday night is starting to look almost vulnerable.

“Sisters” declined another 8 percent from its previous outing last night, averaging a series-low 4.9 rating, according to Nielsen overnights. That was down from a 5.3 last week and 21 percent off the 6.2 it averaged for its Sept. 24 premiere.

It’s also losing a good deal of its “Desperate Housewives” lead-in, though that show is also off from its premiere, down 16 percent over four outings from a 9.5 debut to a season-low 8.0 last night.

Though ABC still led among viewers 18-49 last night, its advantage is shrinking. It’s down from a 6.4 premiere night to a 5.4 the past two weeks, and even finished behind CBS in households last night, when the network got a good bump from NFL overrun and the “60 Minutes” segment about the Duke lacrosse rape case.

But while “Sisters’” decline is certainly alarming, it’s interesting to note that two years ago, “Boston Legal” was drawing even lower ratings in the post-“Housewives” slot, when “Housewives” was drawing better ratings. “Legal” ran through the spring and then was replaced by “Grey’s,” but “Legal” is still on ABC’s schedule two years later.

Thus ABC could stick with “Sisters” and try a new show in the timeslot later this year without necessarily killing off the former. Though a 4.9 is not what you’d hope for after “Housewives,” it’s still helping ABC stay No. 1 this season among adults 18-49.

For the evening, CBS took second to ABC’s 5.4 rating and 13 share with a 4.2/11, followed by NBC at 3.6/9, Fox’s National League Championship Series coverage at 2.1/5, Univision at 1.1/3 and the newly revamped CW schedule at a disappointing 0.9/2.

The CW averaged a 0.9 in its target 18-34s, down 10 percent from the 1.0 the network averaged two weeks ago with the premiere of its urban sitcom block, which was moved to Monday shortly thereafter. Last night “Seventh Heaven” and “Runaway” slipped into those slots.

As a reminder, ratings for Fox, CBS and NBC are approximate, as fast nationals measure timeslot data and not actual program data, and live coverage of baseball and football bled into other timeslots.

At 7 p.m., CBS’s NFL runover and the start of “60 Minutes” led with a 4.8, followed by ABC’s “America’s Funniest Home Videos” at 3.1, NBC’s “Football Night in America” at 2.7, Fox’s local programming and baseball pregame show at 1.3, Univision’s “Hora Pico” at 1.1, and CW’s “America’s Next Top Model” rerun at 0.9.

At 8 p.m., ABC’s “Extreme Makeover: Home Edition” led at 5.5, followed by CBS’s “60 Minutes” and “Amazing Race” at 4.0, NBC’s “America” and the start of “Sunday Night Football” at 3.8, Fox’s NLCS at 2.2, the CW’s “7th Heaven” at 1.1, and Univision’s “Cantando por un Sueno” at 1.0.

At 9 p.m., “Housewives” led with an 8.0, followed by NBC’s “SNF” at 4.4, CBS’s “Race” and “Cold Case” at 3.8, Fox’s NLCS at 2.6, Univision’s “Sueno” at 1.1, and CW’s “Runaway” at 0.7.

At 10 p.m., “Sisters” led at 4.9, followed by CBS’s "Case" and “Without a Trace” at 4.3, NBC’s “SNF” at 3.4, and Fox’s NLCS at 2.5.

Among households, CBS topped ABC, 9.4/15 to the latter’s 8.9/14. NBC was third at 6.1/10, followed by Fox at 5.0/8, CW at 1.5/2 and Univision at 1.4/2.