When Phil came to Vito's house it was to comfort Vito's wife who was also Phil's cousin. She already knew about; that was evident from the dialogue. She was ina sort of denial, so Phil told it it had been confirmed. The question still is how did Phil find out. Given the friction between Phil and Tony over Tony's cousin, etc., Phil's finding out could be significant. I guess it's reasonable that Vito's wife called Phil and told him. Afterall, they are cousins.