An innocent moment of sibling amusement nearly ended in a tragic tumble yesterday for Grace Fennell, 4, and her brother Shamir, 5.

At about 12:45 p.m., the tots were huddled together on top of their bunk beds, watching TV, while their mother was in a nearby room, police said.

Moments later, they were gone.

The youngsters leaned against the screen of a window that was just inches from their bed, and fell two stories to the ground, said an investigator from the police Special Victims Unit.

Grace and Shamir were both awake when medics rushed them to St. Christopher's Hospital for Children. A hospital spokeswoman said last night that Grace was in serious condition, while Shamir was in good condition.

The kids' mother, Monique Tindley, told a CBS 3 reporter that the family had complained for months about problems with the window to the Philadelphia Housing Authority, which owns the family's home on Bancroft Street near Cumberland in North Philadelphia.

PHA spokeswoman Trish Conrad said the agency had no record of complaints and that the window had passed an inspection.

"Obviously, our concern is for the children," Conrad said. "Our thoughts and prayers go out to them." *

Staff writer Gloria Campisi contributed to this report.