She was born in Góra Śląska. Her parents, Krystyna and Zbigniew Kruszyńskis were a refugees from Vilnius. Zbigniew was a soldier, who fought in the Polish resistance during occupation and later continued his career. Sierakowska lived with her family in various places such as Siedlce, Lodz.
In 1970 she joined then-ruling Polish United Workers' Party and was a delegate to the 10th PUWP convention in 1987.
Sierakowska remained a Sejm Member until 2007 elections, when she lost her seat.
During her last term as MP, she served as Vice Chair of the National Defense Committee. Previously she was notable as one of the co-authors of the new constitution, which was adopted in 1997.
Sierakowska left SLD with group of others members led by Sejm Marshal Marek Borowski and founded Socjaldemokracja Polska. SdPl suffered a massive defeat in the 2007 Sejm election and lost all seats, when SLD won fourth place.
She remains one of the SdPl leaders, and ran for Presidency of Lublin, where she has lived for many years, and won first place in the first round with 25,31 percent of vote. Before municipal elections, SdPl, SLD and the Democrats joined forces as Left and the Democrats coalition, a new political power still active. In turnout Sierakowska lost to Civic Platform candidate Adam Wasilewski. Despite defeat, she was elected the same year to the Lublin Voivodeship Sejmik.
In 2007 she returned to the Sejm after being elected from Lewica i Demokraci list.