
Yes the breeder should tell owners of litter mates if one of them develops a serious condition, I can understand why you are angry. Do you know how long she had known?
When my Mums dachsie was diagnosed with a Liver Shunt at 8 months old I made sure she told her brothers owners, she sent me the email they sent back saying he was well but was going to have the info highlighted at the vets, their dam was spayed as it was her first litter, the sire had produced several litters before.