The novel your father was reading was probably
Sword at Sunset by
Rosemary Sutcliff. It is a retelling of Arthurian legend. In the novel the character based on
Arthur is called Artos and the character based on
Guenevere is called Guenhumara, and Hylin is their daughter.
I do not know if
Rosemary Sutcliff invented the name Hylin herself or found it somewhere. However, if she was trying to write an accurate historical novel based on Arthurian legend, she would have been trying to find or create a name that was Celtic, not Old English, and that should be more related to the modern Welsh language than the English language. It looks like she might have based it on the modern Welsh name
Heulwen, which is pronounced "HYL-wen".