By using the ABO blood typing it is possible only to exclude paternity, maternity or brotherhood/sisterhood but it is not possible to conclude positively about these relationships.
For example, if an individual has type O blood, ii genotype, he or she cannot have biological parents of the type AB (IAIB genotype) since necessarily one of his/her alleles has come from the father and the other from the mother. Another example: a couple of individuals of the type O (ii) in their turn can only generate direct offspring of the type O blood, since they do not have alleles that condition antigen A neither antigen B.