>>13774803There is one other possibility, if the mother is possibly heterozygous, meaning she was herself from a mix that had Rh+ and she carries both Rh- and Rh+ alleles, meaning she's likely to produce Rh+ children 50% of the time with an Rh- father or 100% of the time if she is herself homozygous, ie only Rh+, than that's 100%.
In your half sister's case her mother is likely heterozygous and she came out Rh+. The immunological effect in that particular scenario is something I'm not fully aversed of.
http://www.biology.arizona.edu/human_bio/problem_sets/blood_types/02t.html 