Do I need a witness for a real estate contract to be valid?

Yes and no!
No, you do not need a witness for a real estate purchase or lease agreement to be valid in an Ontario, Canada court. The witnessing is simply a way to determine if the person actually did sign the contract. The Consideration is actually the "seal" on the contract. In this case is it is suggested that the seal is initialled through or the signatory actually ends his signature on the seal. This in effect shows the client has sealed the contract and knows what that entails.
Yes, it is always preferred to have a witness for all signatures to prove that the person who signed actually exists adn agrees to the terms they signed for, but it does not negate a contract that is properly sealed.