The blood groups cited by Debbie do not automatically mean that Frank cannot be Ian's father. She only talks about the siblings, which are AB (Fiona and Liam), A (Carl), AB positive (Debbie), 0 (Ian) and B pos (Lip). The genes for blood groups A and B are co-dominant and 0 is recessive. So if one parent had the A and 0 genes, he or she would have blood group A. If the other had the B and 0 genes, he or she would have blood group B. Either can pass on either one of these genes to each child. So genetically, the children can be A0, B0, AB or 00 and their blood groups would be A, B, AB or 0. The only way Frank could definitely be excluded as Ian's father through blood groups would be if one of the parents had blood group A or B and the other one had AB, but we never learn Frank's or Monica's blood group.