Really good question. I asked my brother again today. The TB is diligently sending reminders. Now it gets a bit wild: the homeowner is (still) his mother-in-law, who died 2 weeks ago. The invoice was also issued to her... It’s really an unbelievable story.
I would definitely file an objection in writing. If he really wants to annoy you, he will otherwise pass the claim on to a debt collection agency and then you can deal with the clowns.