Her belongings, including her passport and phone, were found in her room. The first police official said authorities were therefore not able to track her by her cell phone.
Sierra's husband and brother were traveling to Istanbul to follow the search. Her two children, aged 11 and 9, do not know their mother is missing, her brother David Jimenez told the AP Sunday.
Sierra had planned to go on the trip with a friend but ended up going by herself when the friend couldn't make it. She was looking forward to exploring her hobby of photography, her family said.
Crime in Turkey is generally low and Istanbul is a relatively safe city for travelers, though there are areas where women would be advised to avoid going alone at night. The Galata and the nearby Galata Bridge areas have been gentrified and are home to fish restaurants, chic cafes and boutiques.
Associated Press writer Ezgi Akin contributed to this report.