Concerning Cassandra's parents as we don't know their reason of their actions I would not judge them.
Regardless the argument is an interesting one, because if they acted on convictions I would not think they sucked, they would have gambled to offer their kids a better life, and if they acted by ambition, they might have gambled for that as well.
I would think they sucked if they willingly throw away their children lives for sheer ambition, for what I understand of Cassandra, that does not seem to be her opinion.
I hope you don't mind if I disagree a bit.

I will say if the ruler poses some kind of clear and present danger, like some kind of Caligula, there's a moral imperative to try and depose that ruler as soon as possible, which might justify Cassandra's parents gambling their kids' lives. But I don't think the king at the time was a homicidal maniac, as seen by his sparing of Cassandra and Anthony.
I mean, isn't rule number one of being a good parent not putting your kids in a position where there's a good chance they could end up dead? At the very least, if you feel you HAVE to embark on treason, make sure your kids are SAFE somewhere beyond the monarch's reach.
Short of a Caligula-like King, I personally can't spin this any way besides Cassandra's parents were not selfish and negligent AT BEST.
And I don't really remember Cassandra saying much about her parents (except factually) either way, unless there's something I'm forgetting. It's possible she simply doesn't want to rehash painful memories, not that she necessarily thinks positively of them.
I am hoping for a DLC where we go to Nevarra and hopefully explore some of these issues.
ETA: Also, thank you for responding to my rambling, speculative posts. Sometimes a good debate makes you think more deeply about the characters.