I think it comes down to the fact that Cersei & Jaime are twins. Yes, technically one has to be born before the other, but most twins are born close in time together. Leaving out all the incest and hatred stuff, Cersei & Jaime's sibling relationship isn't like Cersei & Tyrion's, simply because she can remember a time when Tyrion wasn't around, and then bam! he was born. I honestly don't think it would ever occur to her that Jaime is her little brother, even though he was technically born after her, because they've been together since Day One.