
It isn't common for it to happen at that age BUT it does. One of my freinds young bitch took it at 7 months after first litter. It is something we ALL have to be aware of when keeping entire bitches.
PS if this is the reason you are looking for other siblings then I wouldn't spend too much time and effort. It is very unlikely any siblings have had this same expereince so young. It also is nothing to do with poor breeding or the breeder in general.