Life leading up to The Sims 3 Pets (console)
In The Sims 3: Pets (console), Lily is a teen sim living with her father, Jamie in Sugar Maple Coast. It is presumed her siblings from The Sims 4 were born before their relocation to the town and are living with her mother, who is unknown at the time.
Life leading up to The Sims 4
In The Sims 4, Lily started as a young adult sim living in Willow Creek with her father, Jamie and her brother, Cody. Her other sister is presumably still living with their mother at this point, who is still unknown. Her father starts dating and soon marries a coworker, their neighbour, Summer Holiday.
She befriends Malcolm Landgraab who had recently became a young adult, and started a romantic relationship with him, and before long they were married. They then had two children, Megan and Alex, but Malcolm is caught having an affair with another woman, his current wife, Ella, who coincidentally has the same name as her sister. The couple divorce and the children stay with Lily.
Soon after, the family move into an apartment in San Myshuno along with their other sister, Ella. Cody (now a young adult) meets, falls in love with, and marries a young woman called Zoe Patel and moves out with her. They have four children, Lily's nieces and nephews, Leo, Lucy, Cedric and Landon.