Two I-rexs?

Yeah, looks like they did.  Then sibling rivalry happened...

The eaten one maybe was D-Rex, Universal has trademarked both names.

Not sure, but it's most likely a dinosaur instead of one of the children because Claire said "it" instead of "him".

@Danielosaurus

do you have a link that has this information? Just out of curiosity...

Perhaps maybe a Mating Pair or a Pack, As so the dinosaur didn't experience depression or such.

Too little to late did they notice that that would in no way be the case. Or, Perhaps, As things do tend to go, The Alpha ate the weaker one.

I'd be interested to see how it plays out. 

Of course, Remember, It's totally in cannon for them to use the death as an Experiment, Referencing the Troodons just kinda being euthenized (Not) in JPTG.

Indominus rex is an extremely important, risky, and expensive genetic endeavor. You don't just make one. You need insurance. If a piano falls on one of them, you can't have all that time and money wasted.

The same reason Jurassic World has plenty of other Tyrannosaurs to pick from, should the big girl in T-Rex Kingdom kick the bucket.

@DUSTYALASKA

I heard it from The Stupendous Wave's channel on youtube.