I got an answer from a complete stranger thanks to our mutual friend
poppinjaye:
Hmmm. This is tricky.
It's highly idiomatic English, and can probably only be rendered by equally idiomatic Latin. There is certainly no way to do this literally.
My first instinct is something like "Noli Sicano adversari de morte" (literally - "Don't oppose a Sicilian concerning death."). It doesn't feel quite right, though.
Perhaps the person who wrote this was on the right track. For "when death is on the line" might well be something like "cum de morte res agatur" ("When the situation is conducted concerning death" might be the closest idiom to "when death is on the line"). But I'm still not sure about "Never go in against a Sicilian" . . . The Romans don't have a command "never!" There is a word for "never," "numquam," but it isn't used in commands, really.
I suppose one could phrase it "Numquam debes" ("You should never," "you ought never to").
Perhaps "Cum res de morte agatur, Sicano adversari numquam debes." ("When the situation concerns death, you ought never to oppose a Sicilian.")
Hope that helps,
Colin Brodd
Teacher of Latin and Classical Humanities
Plymouth North High School
Plymouth, MA
"Qua(e) patres difficillime
adepti sunt nolite
turpiter relinquere" - Bradford Monument,
Plymouth, MA