The others God protected him from were his other siblings who would want to avenge their brothers death. This is also why God Specifically instructed the Israelites to always have a city of refuge for a murderer to protect them from the avenging family until righteous judge could be placed Cain was around 100yrs old and already married when he murdered his brother. Also it states that Cain was the first born son not the first born so Adam and Eve could have had daughters prior to Cain and Abel