FYI, "gene" is the generic term. We're really talking about recessive super-alleles (the version of the gene that leads to having superpowers—in the case of recessive genes, if you have two copies of the super-allele).
For some genetic combinations, penetrance is 100%—if you have one copy of that dominant allele or two copies of the recessive allele, you will display that characteristic, for good or for ill. But there are also genetic combinations for which penetrance varies widely, so that some people with the combination of alleles may display the characteristic in full force while the identical (as far as we can tell) combination of alleles has lesser or no effect on others. There are also other factors that can affect how some alleles are (or are not) expressed—for example, which parent you inherit the allele from, the actions of other alleles, or environmental factors. In other words, if you really want to come up with a scientifically plausible explanation as to why one character has superpowers and a related character does not, you can do it.
FYI, "gene" is the generic term. We're really talking about recessive super-alleles (the version of the gene that leads to having superpowers—in the case of recessive genes, if you have two copies of the super-allele).
For some genetic combinations, penetrance is 100%—if you have one copy of that dominant allele or two copies of the recessive allele, you will display that characteristic, for good or for ill. But there are also genetic combinations for which penetrance varies widely, so that some people with the combination of alleles may display the characteristic in full force while the identical (as far as we can tell) combination of alleles has lesser or no effect on others. There are also other factors that can affect how some alleles are (or are not) expressed—for example, which parent you inherit the allele from, the actions of other alleles, or environmental factors. In other words, if you really want to come up with a scientifically plausible explanation as to why one character has superpowers and a related character does not, you can do it.