What does the expression «the devil himself is no brother» mean?
A brave, decisive, true hero – “ему сам черт не брат” (“yemu sam chert ne brat” or “the devil himself is no brother [to him]”)!
"You know, some merchant's son, a dandy, listened to a lecture somewhere and already thinks the devil is no brother to him," is how Pierre Bezukhov's interlocutor in Leo Tolstoy's novel ‘War and Peace’ disapprovingly remarks about a young man who wrote a leaflet and was brought to trial for it.
This expression refers to self-confident people willing to take stupid risks. According to one theory, the expression stems from the superstition that evil spirits would not harm "their own". Therefore, various house spirits and demons were sometimes considered relatives or even friends. However, they could also jokingly say: "The devil himself is no brother and the pig no sister to him."