If your mother's son predeceases your mother, your mother becomes the sole owner of the property. Joint tenancy does not pass under the Will, so even if your mother's son has a Will, he cannot pass the property to his wife. However, the property is obviously a matrimonial property, and the wife will definitely sue your mother for a share of the property. She will seek a Court Order to sell the property so that she can get your mother's son's CPF monies returned to his account, assuming she is the beneficiary of his CPF monies.
If you want less evil shit, it's better that she adds her name into the property while both co-owners are alive.
If you mother's son predeceases your mother, and your mother becomes the sole owner of the property, when she passes on later the property will go to you unless she has willed to property to somebody else.