There would be chain-of-custody issues if anyone decided to dispute them, certainly.
The article says "Hillblom’s mother [...] had so far refused to comply with a court order to provide blood samples."
So ignoring chain-of-custody issues, if the court has made an order and the mother has refused, some would say it's better the order be enforced by subterfuge than by force.
Could DNA evidence obtained this way be admissible?
Would the defendant estate’s lawyer ask the mother in court if she had provided her DNA for analysis and she would say no?