The Cabaret star is said to have been hiding the painful family secret as her sibling undergoes chemotherapy treatment.
A source tells the National Enquirer, "Liza was absolutely devastated when she first got the news Lorna bad breast cancer. It hit her hard."
The Oscar winner fell out with Luft in 1994 over her substance abuse problems, but they made amends in 2005, when the 60 year old's father, Sid Luft, passed away.
A representative for Minnelli had yet to respond to requests for a comment as Wenn went to press.
The sisters are both the daughters of acting legend Judy Garland. The Hollywood star's father was director Vincente Minnelli.