In The Brightest Night, Sunny holds the Eye of Onx and it doesn't kill her, so my theory is that either:

1. It saw Sunny as a good possible heir

Or

2. It somehow knew that Sunny did not want to be queen

Because Blister seized the Eye in greed and it killed her, so I'm leaning on the side that it saw Sunny as a possible heir, because as far as we know it was not enchanted to read minds.