Wilmington police responded to the report of a stabbing in an apartment above a church on the 700 block of East 11th Street at around 8:45 p.m.
Authorities say the man climbed out a window onto a landing to escape the assailant who authorities identify as the victim's sister.
The victim was stabbed multiple times in the chest, police say.
The victim was transported to Christiana Hospital in critical condition.
Police have taken the sister in for questioning and charges are expected.
Wilmington police continue to investigate.