Question: “My mom is thinking of transferring her principle residence into a joint tenancy with my brother who will also live in the house. Will there be any Ontario Land transfer tax on the development of the joint tenancy?”
It does not “appear” that the land transfer tax applies in this case. See:
A call to the lawyers office who will handle the transfer will provide some certainty to the situation (I’m not a lawyer! I’m an accountant).
In term of the transfer, joint tenancy carries some risks other than tax risk. Take a look at this article on the dangers of joint ownership.