Are you saying he's breeding brother to sister, or just remarking he has them?
Don't put all your eggs in one basket... bitches miscarry, have one pup, lots of things.
As for whelping, I thought you said breeding and had to reread. The southern hemisphere is about 6 mos later for estrus. The below paper, I wonder if they followed up to see if basenjis are losing seasonal cycles.
For the most part, estrus occurs within the months of September and October for Basenjis living in the Northern Hemisphere. Breeders have also noticed a second estrus sometimes occurs around March or April. In England many Basenjis have a second estrus (Burns, 1952). This has created some concern that the Basenji is losing its annual cycle since leaving Africa. The tendency for a second annual cycle has existed in Africa and in the Bitches imported from Africa....