ANSWER:

They don’t become kafirs for not talking to one another! 

Prophet (peace and blessings of Allah be upon him) said: “It is not permissible for a man to forsake his Muslim brother for more than three days, each of them turning away from the other when they meet. The better of them is the one who gives the greeting of salaam first.” (Narrated by al-Bukhaari, 5727; Muslim, 2560). 

QUESTION: Sheikh is there any authentic hadidth that says if two brothers don’t talk in a row for three days they become kafir