I would say don't tell the family. In my situation it hasn't done any good. The one rational thinking in-law that I have in my sitch doesn't have the power to do or change anything, so don't waste your time.
I'm about 5 months into my separation and just found out 2 days ago for sure that my W is having an affair.
The best advice I can give is to spend as much time with your son. Possibly take legal action to get a joint care arrangement if possible... currently you are setting things up as her being the primary care-giver and may be phasing yourself out of your son's life.
It is extremely difficult to follow the DR advice, but the sooner you get to the acceptance stage instead of the denial, anger, depression, pleading/negotiating (if I do this, she'll do that).... the sooner you will find yourself in a good place in your situation.
