Sigi started playing basketball in Austria and being a really young player, she moved to the United States to study and play for Stearns High School, where she averaged 23.8 points, 10.8 rebounds, 4.7 assists and 3.2 steals per game in the 2011/12 season.

Sigi Koizar started her college career during the 2013/14 season, playing for Maine and averaging 4.9 points, 3.8 rebounds and 2.5 assists in 20 minutes per game.

In the 2014 summer, Sigi Koizar played the 2014 European Championship for Small Countries with the Austrian National Team and she averaged 15.4 points, 5.6 rebounds and 6 assists in 23 minutes per game, remaining undefeated and shining against Iceland with 32 points and 10 rebounds.

Sigi Koizar kept improving and already in the 2014/15 season she had her first "big" year at Maine, averaging 14.3 points, 4.2 rebounds and 2.7 assists per game and being named in the AEC 1st Team.

In the 2015 summer, Sigi played the U20B European Championship and she performed well, averaging 12 points, 8.8 rebounds and 2.8 assists per game.

Sigi Koizar had a fantastic 2015/16 season with Maine, being one of the best scorers in the country with 17.7 points, 4.1 rebounds and 3.8 assists per game with excellent 43.4% from 3 point range. In her junior year, Sigi was named again in the AEC 1st Team also she also reached the AEC Final.

In the 2016/17 season, Sigi Koizar had a great last year and she averaged 14.1 points, 4.5 rebounds and 2.4 assists per game, being named for the third time in the AEC 1st team and reaching the AEC Final one more time.

During her 4 years at Maine, Sigi Koizar was also a very good student and she was named in the AEC All-Academist First Team in 2015,2016 and 2017.

In the 2017/18 season, Sigi Koizar started her professional career and signed in Hungary with ZTE NKK and she reached Hungarian League Semifinals being a main contributor with 13.3 points, 4.6 rebounds, 2.9 assists and +16.6 efficiency per game with 40.2% from the 3 point range.

Sigi re-signed with ZTE NKK in the 2018/19 season and she had again a sensational year reaching Hungarian League 3rd position and averaging 10.8 points, 5 rebounds, 3.6 assists and 16.4 efficiency per game with 53.5% from the field and 38.3% in 3 pointers.

In the 2019/20 season, Sigi Koizar signed in Russia with Enisey Krasnoyark and she averaged exceptional 11.1 points, 5.5 rebounds, 5.2 assists and +16.2 efficiency per game in the Eurocup and 8.9 points, 4.1 rebounds and 4.3 assists per game in Russian League.

In the 2020/21 season, Sigi Koizar signed in France with Charleville and she averaged 3.7 points and 1.7 assists in 13 minutes per game.

In the 2021/ 22 season, Sigi Koizar signed in Spain with Leganés and she averaged 2.5 points, 1.7 rebounds and 0.9 assists in 13 minutes per game.

In the 2022 summer, Sigi Koizar signed in Australia with Logan Thunder and she averaged 10.8 points, 4.8 rebounds, 5.5 assists and +15.8 efficiency per game.

Sigi played in Italy, with Valdarno, during the 2022/23 season and she averaged 4.7 points, 2.5 rebounds and +5.2 efficiency per game.

With the Austrian national team, Sigi played the Qualifiers for the 2023 Eurobasket and she averaged 10 points, 4.8 rebounds, 3.8 assists and +11.3 efficiency per game.

In the 2023/24 season, Sigi Koizar signed in Romania with Constanta and averaged 8.4 points, 4.8 rebounds and +10.3 efficiency in 22 minutes per game in Romanian League and 8.2 points per game in the Eurocup, shining against Galatasaray with 24 points and +24 efficiency.