joekhoo,
I'd really like to talk you out of breeding...
The Breeder you got your pups from, is experienced apparently, and has a ample supply of Dogs available. I have a similar situation as you, my Male and Female are Brother/Sister, BUT, mating them is NOT an option. Just TOO risky for their offspring to be effected.
Please rethink this, have both altered, and remember that those animals are going to live maybe 15 or so years. They'll be around for a long time.
My Male was 2 years old, when I got my Female, and in about another 2 years, I'm thinking about getting another Male. This way, they're sorta staggered in age, and when the time comes, there will be some that will remain, when the oldest passes.
Regardless of if you have your second animal from the same Sire and Dam, I would suggest that you choose the opposite sex, for the first one's companion. Seeing the reaction of that between them, and I'm not talking about sex, is something... a love, which just can't be compared. I witness this each day, I see my two interact.
So...
Same Breed.
Opposite sex,
A little separation in their ages,
BOTH altered !
Have a good Day !
S.W.