Tel Aviv police on Thursday arrested the 18-year-old sister of a young woman murdered in the city earlier in the day, on suspicion of being involved in her death.Hours after the murder the sister and her boyfriend confessed to the crime, police said, though they added they have yet to determine a motive.The victim, described as an 18-year-old woman, was found stabbed to death in the entryway of her family home in a residential building in the quiet Nahalat Yitzhak neighborhood.Police received a call at around 12:30 p.m. reporting the stabbing, and arrived to see the woman showing no signs of life.Magen David Adom paramedic Iko Menashe said, “When we arrived at the scene we found a woman who was around 20 years old laying lifeless with signs of serious violence on her body. We attempted to give her first aid, but her injuries were very grave and we had no choice but to pronounce her dead.”Within minutes of the crime police said they were able to rule out a nationalist motive for the crime, which they determined was criminal and not terrorism.Police said it was the victim’s sister herself who called police to report the stabbing, and that she said that she was with the victim inside the apartment when she stepped out to the stairwell and began screaming. The arrest of the sister and her boyfriend was made after there were contradictions found in the story she told police.Police said they found the boyfriend still at the scene of the crime, and decided to take him and the sister into custody on suspicion of being involved in the killing. Neither he nor the sister resisted arrest, police said.A spokesman for the Tel Aviv police said that there is no history of domestic violence or police complaints filed by either sister. However, they did receive testimony that the two had a number of disputes in the past, though nothing that would give reason to suspect a murder would take place. At the moment they are still trying to determine what possible motive the sister and the boyfriend may have had for killing her.Both suspects will appear in court on Friday in Tel Aviv for a remand extension.