Fundamental problem here is that DNA mutation is not the reason siblings are not identical.
Grossly simplified explanation follows.
We each have 23 pairs of chromosomes in each cell, subject to some important exceptions. Chromosomes are essentially very long DNA molecules. The chromosome pairs can be reliably differentiated from other pairs, and are numbered 1-22 and XY. It is possible to locate sites on chromosomes where a given section of DNA codes for a protein. That site is a gene. It will appear at the same place on both versions of the chromosome in all people. Because the chromosomes come in pairs, there will be two versions of the gene in each person. One version will have come from Mum and one from Dad. The two versions may well, of course, be coincidentally the same (in which case the owner of the genes is “homozygous” for that particular gene).
Now we come to one of the “important exceptions”. When a sperm cell (or for that matter, an ovum) is created it only has 23 individual chromosomes, not 23 pairs. If this were not the case, chromosome numbers would double in each generation, rapidly resulting in an unfeasibly large number of them in each cell.
The important issue is how these individual chromosomes in sperm cells are created. A process called recombination occurs whenever a sperm cell is created, whereby information that had been on both components of a pair of chromosomes is shuffled around. Some information might come from the “first” of a pair of chromosomes, some from the “second” - it’s all fairly random. The amount of information in a sperm cell is half that of the information in a regular cell, but how that halving is achieved is essentially random.
The result is a sperm cell that is essentially unique, because of the large number of possible ways of combining all the various paired pieces of information in a person’s genome. The process is called meiosis.
You can see this in a small way when you recognise that some sperm are X and some are Y, otherwise all children would be the same sex (ova are all X, because all women are XX)
Thus, all the sperm cells in the body of Dad are related (because there is only a finite pool of information from which they can have come) but are nevertheless each unique because of the recombination process that is part of meiosis. This is because of the huge number of different ways the process of “this gene from locus so-and-so on chromosome pair 21 and that gene from locus so-and-so on chromosome pair 16” can occur. The same is true for the ova in the body of Mum.
And that is why brothers are all different, as are sisters. Dad’s body mixes and matches the DNA from his parents as does Mum’s in such a way that means that Mum and Dad’s children will be related, but unique.
If it were not for recombination, all siblings would be mostly identical.
If the cause of differences between siblings was DNA mutation occurring between each birth, then the genome would be so unstable as to be unworkable.
DNA is robustly resistant to change. Its almost magical processes of self-replication have error checking mechanisms. Mutations that do occur in the genome are (relatively) rare. Those mutations which translate to viably coding for proteins are rarer still. Those mutations which code for proteins that are compatible with life in a viable offspring still rarer. Evolution requires a balance to be struck between the generation of a reliable, stable genome (so that we have something stable to pass on to the next generation) and having some modest capacity for change (so there is sufficient variation in the species’ gene pool to allow for selection to take place).
So…looking at an astronaut’s offspring for genetic change in the way you suggest is not likely to be productive, because all offspring are genetically unique for reasons that have nothing to do with mutation. Of course, a mutation may occur as a result of space flight, but it has to have affected a sufficient number of sperm (there are 10s or 100s of millions in an ejaculation) to result in noticeable change which is capable of being distinguished from non-space related mutagens (astronauts, and the rest of us, are exposed to mutagens on earth all the time).
She’s, um, shorter than I expected.)