The 2018/19 season was very successful for Sieff. So she started the summer in the Youth Cup in Oberstdorf with a second place behind Ema Volavšek . A few weeks later she won the ski jumping competition FIS Carpath Cup in Râşnov . After Sieff was able to achieve two victories and three further podium finishes in the Nordic Combined Alpine Cup, she finished the overall ranking of the Alpine Cup in second place. Since the jumps are integrated into the competitions of the Alpine Ski Jumping Cup , she also regularly competed in ski jumping and came ninth in the overall ranking. At the Nordic Junior World Ski Championships in Lahti , Sieff finished thirteenth in the combined junior women's competition, which was held for the first time. In addition, she competed in the special jumping, where she reached the 36th place in the individual and seventh place with the team.
In the summer of 2019, Sieff initially started again in the Alpine Cup competitions in ski jumping and Nordic combined. She took second place in the summer tour, which is integrated into the Alpine Cup, both twice in ski jumping and in combination. Because of these successes, she was nominated for the Grand Prix of Nordic Combined 2019 despite her age of 15 . On August 24, 2019 Sieff won the first mixed competition in the Grand Prix in Oberwiesenthal together with Alessandro Pittin , Veronica Gianmoena and Samuel Costa . A day later, she also made her debut in the individual in the Grand Prix and immediately finished fourth. She finished seventh in the overall Grand Prix standings. The Youth Olympic Winter Games in 2020 in Lausanne took Sieff sixth. In addition, she won the bronze medal in the Nordic mixed team competition. Around a month and a half later, she finished tenth at the Nordic Junior World Ski Championships 2020 in Oberwiesenthal using the Gundersen method , making her the best Italian. In addition, she and her teammates came fifth in the Nordic Combined mixed team competition.