To be clear: I'm only referring to John Paul (Father), Theresa (Mother), Peter, Valentine, and Ender. Who has what we would call the highest "I.Q"? I'd much prefer an official quote from the books or Card, but inference from the books would be interesting too. If possible, try to establish an "order" of intelligence.
In Ender's Game, Valentine says:
Ender and I aren't stupid. We scored as well as you did on everything. Better on some things. We're all such wonderfully bright children. You're not the smartest, Peter, just the biggest.
Peter agrees with her, so it's presumably the truth. It seems reasonable that the "everything" includes some sort of I.Q. test, so this indicates that Valentine and Ender score better than Peter in an I.Q. test.
(John Paul and Theresa were given permission to have Andrew because Peter was too vicious and Valentine too mild, with Andrew "just right", rather than because of anything to do with intelligence - see the musing by Valentine in Xenocide).
There's not a great deal of information about Ender's parents, even in the First Meetings stories "The Polish Boy" and "Teacher's Pet". We know that John Paul scored very highly in leadership, and must have had reasonable intelligence as well, otherwise Captain Rudolf (of IF) wouldn't have been interested in him (and he did graduate with valedictorian honours, after moving to America). However, the implication is always that it's Ender's ability to understand human (and non-human) nature that was his father's gift.
In "Teacher's Pet" and the Shadow books, we see that Theresa is "a genius researcher in her own right", but there's no measurement specifically; we do know that Ender's (and Peter and Valentine's) maternal grandfather was a "renowned" genius. The implication is that the children's gift for strategy comes from their mother.
In Ender's Homecoming, John Paul says:
Theresa, you're the smartest person I know.
And we see the children referred to as "super-geniuses". So if you want a rank, it would be John Paul, Theresa, Peter, then Andrew and Valentine in some order or perhaps equal.