Exactly how bad is it to call a family member by their given name?
Calling a family member of the older generation by the first name only is taboo. You must address your parents and grandparents with appropriate honorific or pet name e.g. /爸爸，媽媽/ 公公，奶奶
Calling a family member in your generation who is older than your (e.g. older brother, older cousin) by the first name is also not natural.
For uncles and aunts, you can use their first names but must add their titles at the end. e.g. 二叔/ 志強叔，三叔/ 國強叔
You are free to call any family member in your generation who is younger than you by the first name, except if one of your uncles is younger than you, you still have to treat him as your senior.
You are free to call any family member of the younger generation by the first name. However, it is considered too formal. Most people use pet names to call the younger family members. e.g. 小明 instead of 家明