Well said Mir, and to add to this it's really simple and costs nothing but a few seconds to prove you owned a granny.
Click your cat, open pedigree, it will say breeder YOU on the relevant granny. Take screenshot or gyazo and show the auctioneer.
Cobreeders dont need proof, if someone is your partner, they can own a granny by default

Grannies in law lol
Example proof. Note Vee owns the granny, I clearly bred it.