If you go to the edit tab on the father, it will list all of his relations on the right hand side. Under his wife's name, there will be an option to add a spouse. Use this to enter the other wife. Make a note of her profile code. Now visit the edit tab for the son. Under his relations, find the mother. There is an option beside her name to edit her information. On that page, it asks if you would like to replace her with an existing WikiTree profile. Enter the profile code for the correct mother. Now your changes are complete!