Q: Did Jade eat his siblings?

The theory goes that Jade and Floyd hatched from eggs (moray eels can lay up to 10,000 eggs at once in real life, though in-game is unspecified) at approximately the same time.

Jade then selected Floyd as the one sibling he would spare, and ate the rest. The reason why he chose Floyd is technically not specified.

This is based on many things that can be found throughout the game, such as this cryptic comment from Jade: “I’m glad I chose you as my partner when we were but little elvers.”

Floyd responds, “Not sure what that smile’s for, but I’m glad we survived together, too,” which may insinuate that if something did happen, Floyd might not know what it was.

Jade also says, “there are five in my family at present.” One interpretation of this line is that their family used to be larger, and might get even smaller in the future, but five is where they are at now. 

There are some who theorize that maybe their mother is pregnant and there will actually be a new addition to the family soon rather than a loss, but we have been given a surprising amount of information about how common it is for people to go missing in the Coral Sea, with no hints that they will soon be getting between 1 and 10,000 new siblings.

Well, we live closer to large carnivores and venomous creatures than people tend to on land.
You never know what’s lying in wait around the corner. You must keep vigilant at all hours.
They would tell us which areas to avoid and what times were safe to walk around. Yet every year, a few students inevitably went missing.

Floyd has confirmed that they hatched rather than being born like mammals, and there is an additional theory that they even hatched from the same egg, which is why neither twin is older than the other.

(While EN specified “hatched outta our eggs,” plural, Floyd’s original dialogue did not, so it is possible that they either hatched from either separate eggs or the same one.)

In Floyd’s dream he says that he is used to having “a body double around” and Ortho assumes that he means Jade, but Floyd then says, “Jade and I don’t look alike at all.”

Idia responds, “Ortho, that sounds like the kind of line where getting the full explanation leaves you horrified. I wouldn’t pry if I were you!”

Another point in favor of the “Jade ate his siblings” theory is how, on the subject of ghosts they have seen, both twins mention seeing people that looked strikingly like each other, only to realize that they weren’t. 

Floyd: “I once thought I saw Jade in three different places at once.”

The theory goes that they saw the ghosts of their dead siblings.

This may or may not be considered evidence of how the twins might have had other siblings at one point and something happened to them, but even if so, it could have just been a Finding-Nemo style incident with a barracuda or something similar.

So why do people point to Jade as the perpetrator?

Maybe irrelevant, but Rook’s nickname for Jade in the original game is, “Monsieur Premeditated Crime.”

Jade is a heavy eater, on par with Sebek (another thing they have in common is they have both threatened to eat Grim), saying that people are often surprised by how much he eats.

Jade says this is because his “fuel efficiency is lacking” (low blood sugar?).

Floyd is aware of this and seems to go to extra lengths to make sure Jade eats properly, encouraging him to relax and fetching food for him during Halloween.

To the original question: no, there is not a definitive line in the game that states “Jade ate his siblings” that we can point to as proof that it actually, canonically happened.

But we do have many cryptic lines that might possibly be insinuating that a infamously hungry Jade chose Floyd as the one sibling he would spare and ate the rest, Floyd may not know it happened, and Jade might be actively choosing not to tell him.