Anyone named Ennis would be plagued with being called Ennis the penis, especially in the lower grades at school.It sounds like you are looking for Scottish names , oldest brother is Fergus and the younger is Angus.
Thanks. I think that one of the brothers may not even show up in this novel, so I'm rethinking the names and their backgrounds. Not sure if the father having a dead twin brother twin may add an unnecessary plot twist.
Since Ennis rhymes with Dennis, I have to say I never considered the penis reference (I wonder if boys named Dennis have that problem).
