Complex sentence: Soderberg was visiting London at the time of the June 2017 London attack, traveling on to Paris with her 16 year-old niece where the two were locked down inside the cathedral during the 2017 Notre Dame attack.
Split into simpler sentences: Soderberg was visiting London at the time of the June 2017 London attack. There, the two were locked down inside the cathedral during the 2017 Notre Dame attack.