Let's look at the blood types one factor at a time and see what it says about paternity of the father.
Rh factor - this is a simple dominant trait with essentially a dominant and recessive allele. The son has to be homozygous recessive but the girl could be homozygous dominant or heterozygous. If the two parents are both heterozygous, they could have both offspring so this does not address paternity.
ABO blood type - this is a three allele trait with O recessive and A and B co-dominant. The son must be homozygous recessive (oo) and the daughter must be heterozygous co-dominant (AB). If both parents are heterozygous with the recessive allele (father Bo) and mother (Ao) they could have both these children so this also does not address paternity.
MNS - this is a more complex two gene system but the M and N alleles are co-dominant within the system. Since the daughter is MN, at least one parent must have an M allele and an N allele (although one parent could be MN while the other could be M or N). In either case, neither the father or mother has an N allele which would argue against paternity by this father.
In the simplest case this would be argued that the so-called father is not the father of the children because they are twins. However, they are fraternal twins and there are numerous instances of fraternal twins having different fathers so you can really only say that the daughter is not the daughter of this man.
Just to be really real world, we now know that not all alleles always express, a phenomena called incomplete penetrance. I don't know if this is the case for the MNS blood group or not but this is part of the reason we have moved to DNA fingerprinting for paternity testing as it is more reliable.
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