This could happen in the next several years.
In this story, though Eilonwy did not appear, we come to understand the ultimate fate of Eilonwy's mother, Princess.
Eilonwy's father was a commoner; her mother Princess Angharad was evidently never fully trained as an enchantress.
Depending on the truth of these matters where i have no clue and how well they can be explained here, some other articles should link to this section.
The Chronicles concludes with an announcement to the people of the coronation and marriage of King Taran and Queen Eilonwy.
This would make her an orphan - just like Taran.
That, of course, is speculation on my part.
He is an annoyance to until they spend more time together as frogs, and he becomes her love interest and later husband.