Siderius had been a staff employee for the Socialist Party fraction in the House of Representatives before she became a temporary member of the House herself. She was member of the municipal council of Zwolle between March 2006 and September 2014.[2]
During the 2012 general election Siderius was number 18 on the Socialist Party list and received over 3000 votes. The Socialist Party conquered 15 seats and as a consequence Siderius was not elected.[3]
From 6 February 2014 Siderius temporarily replaced Renske Leijten who went on maternity leave. When Paulus Jansen resigned in April 2014, Siderius became his permanent replacement on 14 May 2014.[5]Henri Swinkels then became the temporary replacement for Leijten.[6] Her term in the House ended on 23 March 2017.[2]