This is not an uncommon problem - some bitches do not get along with their offspring after the offspring reach adulthood. My parents' neighbor bred their Husky bitch, and kept one of the males in the litter. The mama and son are very attached and never fight with each other. However, the owners tried to have a 'reunion' with one of the other littermates, when the 'puppies' were about a year old. The mama and son ganged up and attacked the other son, who had to be rescued and taken away by his owners. The family ties often do not mean that the dogs will be friends!
My Boo has a littermate sister who was also adopted into our area. When the two of them get together, they instantly growl and snarl - there's sibling rivalry for you!