You can't divorce your son and daughter-in-law.
It is their marriage, so only they can get divorced.
If your son's wife is filing for a divorce against your son, then he must defend the divorce and tell the court how the assets must be shared. This depends on whether they are married in community or property, or out of community of property, or out of community of property with the accrual. You can read up more here.
If you gave your son R5000 to assist him make ends meet, you'll probably never see that money again.
If you gave your son R70,000 as a deposit for their house, then you must claim that it was a loan. That means that the R70,000 is a debt in your son's name, and that will be shared by his wife! That's good for you and will reduce what his wife gets from the marriage.
I suggest that you tell your son to speak to a divorce attorney to assist in defending the divorce, and to represent his interests in court. I am afraid that I am not allowed to refer you to a specific attorney, however I can suggest that you can locate a suitably qualified attorney in your area who will be able to assist you with your query at www.southafricanlawyer.co.za
Your question was "Can you divorce your son and daughter-in-law" and my answer is "no". But I've explained what you can do instead!
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Good luck and best regards,