... I don't believe I'm speculating on the future direction of property prices when I say that selling a property right now if you don't have to isn't the best idea.
Of course you are speculating: you are making a decision based on your opinion of future prices. And that is what OP and his brother want to do. They are, of course, entitled to do so, but they should make their decision while being mindful of the danger that property prices might fall further.