The novel your father was reading was probably Sword at Sunset
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".