which film was her breakthrough?
"Heavenly Creatures"
Kate Winslet
"Heavenly Creatures" 
who played kate winslet's mom in titanic Frances Louise Fisher is kate an actress? Yes, she is
wiki:5370381
Kate Winslet [SEP] 1994–1996: Film breakthrough
Winslet was among 175 girls to audition for Peter Jackson's psychological drama "Heavenly Creatures" (1994), and was cast after impressing Jackson with the intensity she brought to her part. The New Zealand-based production is based on the Parker–Hulme murder case of 1954, in which Winslet played Juliet Hulme, a teenager who assists her friend, Pauline Parker (played by Melanie Lynskey), in the murder of Pauline's mother. She prepared for the part by reading the transcripts of the girls' murder trial, their letters and diaries, and interacted with their acquaintances. She has said that she learnt tremendously from the job. Jackson filmed in the real murder locations, and the experience left Winslet traumatised.