The Rh factor information doesn't discriminate potential fathers of the child. Even though the child is Rh-, it is not necessary that either parent be Rh-.
The father of the child could be either Rh+ (heterozygous with one Rh+ allele and
one Rh- allele) or Rh- (homozygous with two recessive Rh- alleles. )
When the mother is Rh- and the father is Rh+ (heterozygous with one Rh+ allele
and one Rh- allele), there is a 50% probability for an Rh+ child and a 50%
probability for an Rh- child.
When the mother is Rh- and the father is Rh-, as in the case of your own family,
then all children (all of your siblings) will be Rh-.
Once again, if it is important to have correct and highly discriminating
information about paternity, my advice would be to explore the possibility of a
DNA-based paternity test. Although I cannot personally endorse any particular
testing company, you may wish to visit the web page of a paternity lab at