Hello and welcome to COL
Many vets will castrate from about 6 months onwards, but many also advise to wait until at least 12 months old or older to allow the puppy time to mature physically and mentally. Some recent scientific studies have shown evidence of a higher incidence of physical problems such as cruciate ligament damage following neutering at a young age.
My own preference is not to have my male dogs neutered, but I can understand your worry about yours being around your sister's unspayed female. If you choose not to have him neutered until he's a bit older does your sister know roughly when her bitch is due her next season? Would you be able to make temporary arrangements to keep them completely separate during that time to give your puppy a bit more time to mature before you have him neutered?
One of my 2 males is entire, the other has been neutered because he had an undescended testicle (increased danger of testicular cancer in later life) but I waited until he was 15 months old before having him neutered in order to give him time to mature.
