This is a tough one, on various levevls.
I do entirely sympathise with your feelings with regard to your sister. I vividly recall a woman i was deeply in love with starting a relationship with my best friend just eleven days after we broke up. I could have accepted (still devastated but accepted) her seeing someone else, but did it have to be someone I was close to - sleeping in a house i knew so well, removing my main source of support?
I know this must be that feeling to the nth degree because this is your sister, and you perceive that she has deceived you.
It won't stop the pain, but consider that she kept thsi situation from you in an attempt to avoid being the cuse of any more hurt for you. Obviously this has failed, but her motives may have been of the best, flawed and illogical though they were. But we are all human, and we do things we think are right, even if it is obvious that we are simply delaying the inevitable, it still feels better than jumping in and causing you grief up front so to speak.
Similarly, we don't choose the person we fall in lovv with. It is easy to say that your sister should have 'kept away', but that wouldn;t have brought you and your ex. back together - and it is easy to take the moral high ground on this aspect of the situation.
As far as your son is concerned, children are very resiliant at this age, he will accept the realities of the situation far more easily than you may think - although he will know you are hurt, even he doesn;t completely understand why.
Try to see this as a situation that has happened, but not one which has been designed and plotted to attack and hurt you. It will take a long time to get used to the new relationships for everyone, but you must try and stay calm for your own well-being, as well as that of your son.
My heart goes out to you.