The terrified sister of a 26-year-old Queens woman who vanished Sunday believes her deeply religious sibling was murdered — but cops are not convinced.

Gurpreet Kaur, 23, received a bizarre text message from her sister Rajwinder’s cellphone at 9:40 p.m. that day, saying, “The girl with this phone is dead was smiling.”

“It was a psychotic message,’’ said Kaur. “This should be treated as a murder.”

Rajwinder was last seen leaving the family’s Hollis home to do volunteer work at a Brooklyn homeless shelter.

Gurpreet said it would be out of character for her sister to send such a crazed message. “It was so creepy and very dark and foreboding, and I was scared,’’ she said.

Asked if Rajwinder may have joined a cult, Police Commissioner Ray Kelly replied, “That’s in the equation; it’s something we’re looking at.’’