Suppose you create a number of pre-embryos for use in IVF. Imagine you transfer one to a woman’s womb this year and it results in a live birth. The remainder of the pre-embryos are cryopreserved. A couple of years go by and you transfer a second one into the same woman’s womb. Again, a live birth results. Are those two children twins? (I don’t think anyone will argue they are identical twins. If they are twins, they must be fraternal twins.)
Now I have to say, the answer seems so obvious to me I wonder at the question. Clearly they are not twins (as far as I am concerned.)
But perhaps others will reach a different conclusion. So let me unpack my thinking. Genetically speaking, the two children stand in the same relationship to each other as any children with common genetic lineages. This is true for all fraternal twins. So then what distinguishes fraternal twins from siblings?
I think for me the answer would be the shared time in utero. If there is something significant about that time, then it matters that for fraternal twins it is shared time. These two children do not have any shared time in utero and thus, to me, it makes no sense to call them fraternal twins.
Of course, if you think the critical thing is that fraternal twins are conceived at about the same time, then these might well seem like fraternal twins. Which is why I think this is a bit of a Rorschach Test: Your answer to the twins questions says something important about other things that matter to you.