First of all, what is the source of information claiming to know the name of Emma’s brother? I doubt that would have been spoiled to the press if it’s such a significant name. Yet, assuming the baby is named Charles, that doesn’t mean the baby is named after Hook. Charles is not his real name; Killian is. What if Charles is the name of Charming’s father, for example? That could explain why it would be meaningful to Snow and David. The fact that Emma calls Hook “Prince Charles” in the sneak peek means it was just the first name to pop in her head. Maybe Emma is inspired to use the name that her parents just chose for her new brother. Or maybe it’s a foreshadowing of what Snow and David decide to call their son. I think it’s most likely that Emma was inspired to call Hook Prince Charles though because she is from our world and knows that Prince Charles is the Prince of Wales in the UK.
"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy