Most poskim do not admit blood tests as evidence that a husband is not the father of his wife’s children and cause mamzerus issues. I have no idea where the fathers were (or weren’t) in the story but one of them probably objected. Particularly if you understand that the Posuk calling the women zonos is to be understood with the usual meaning.
According to the other opinion mentioned in Chazal that they were mother in law and daughter in law pair , both husbands were dead and it was a question of the DIL doing Yibum. If so what would a DNA test have helped? Either way it would show relationship with the MIL . She was either the baby’s mother or grandmother. (the MIL was in fact the actual mother)