
Your sister's dog may show no interest at all in your bitch, vague interest or go bonkers - it's impossible to predict. If he was castrated more than a year ago then at least if they do mate (it can happen) no pups will result.
The best time to spay your bitch wil be three months after her season, as long as she's not having a phantom at the time. A good way to minimise the chances of this is to start her on raspberry leaf tablets (available from pet shops or health food shops) now, and continue them for several weeks.
:)