From Formspring: Would a dragon ever eat its mother?

Technically, this is impossible. Dragon's are living conduits of elemental power. When they die, that power consumes them and they break down into ash. This takes about an hour or so, depending on the size of the dragon. It's why there aren't any fossils or bones left.

Even if it could, I doubt a dragon would ever eat another dragon, much less it's mother. Dragons are exceedingly speciest. Even ones who are benevolent to humans still tend to consider them lesser beings. Kind of like how vegans view animals. Sure, vegans like cows enough not to eat them or make clothes out of their skin, but they wouldn't marry one or let it vote. Dragons do sometimes eat their enemies. In the first book, there is a scene where Meg discusses dragons eating people and is somewhat shocked to learn that Garrett, in dragon form, has eaten people. He replies that he didn't do this for fun, it was purely a tactical move: chewing someone up and swallowing them is a fast, effective way to end a fight. He then goes on to explain that humans taste terrible...like apes.

For a dragon, eating an enemy is an insult, like saying, "I'm so much bigger and better than you that I can effing eat you and there isn't anything you can do about it!" so it is unlikely that a dragon would insult another dragon in this way. Even dragons that hate each other have enough species solidarity to make such an act unpalatable--pun intended. But swallowing a demon or a human is no big deal, unless it's someone the dragon likes.