Why must a mother's dna match that of her biological children? why would damian expect half of the children's dna to come from their mother? why not 30% of it? or 70% of the dna?

Because each parent contributes to their child 23 chromosomes, so therefore each parent gives their child half of the DNA of the child.

It wouldn't be 30% or 70% because the balance of the chromosomes the parents contribute is always equal.

Because that's how it normally goes