By Tony Hanson, Dray Clark, Steve Beck and Dianna Rocco

PHILADELPHIA (CBS) – Investigators believe a Philadelphia woman killed her twin babies and attempted to kill her young daughter and then herself because she thought the children’s father was having an affair with a family member.

Authorities charged Stacey Smalls, 41, with two counts of 1st degree murder Friday afternoon for the deaths of her 18-month-old twins, a boy and a girl. Police believe Smalls strangled the one twin and drowned the other. Authorities are awaiting autopsy results for the official cause of death.

The horrific discovery was made Thursday afternoon inside a home in the 6300 block of Ditman Street in the city’s Tacony section.

In addition, police say she gave her four-year-old daughter some type of substance to drink in a suspected attempt to take her life.

The four-year-old was taken to St. Christopher’s Hospital for Children where she is listed in stable but guarded condition.

Police say Smalls, who neighbors say worked as a nurse at an area hospital, then attempted to take her own life by cutting her wrists. She remains in the hospital.

Police sources say Smalls left a note at the home addressed to the children’s father letting him know that she did what she is accused of doing because she learned that he was allegedly having an affair with a family member. Police say the father is the one who made the discovery and called 911.

Police questioned him earlier Friday. The father has asked for his privacy as a memorial to the children grows outside the house. Friends and coworkers have stopped by to show support.

Philadelphia homicide detectives are investigating.