How many guests are coming? If it's a large party (eg 50 guests), it might be awkward and inconvenient to cancel the whole thing. Not to mention, it would be unfair to those guests who love you and DS and want to celebrate this occasion.
My next question is: can your BIL, SIL and DH's friend be trusted to put aside their differences for one day? Or alternatively, decide that one of them will not attend? If so, I recommend doing nothing.
However, if you suspect there will be unpleasant drama, I'd definitely dis-invite some or all of them. If BIL is DH's brother, it's easy. Disinvite the friend. Some good wording might be "Friend, my brother will be attending DS's birthday party. Given the circumstances, we don't feel it's appropriate that you attend.."
If SIL is DH's sister, it's a little trickier. Is she now officially in a relationship with DH's friend? If she is, and BIL plans to attend, I think you can ask her not to bring DH's friend along (and/or tell DH's friend yourself).
Alternatively, if you don't want her there at all, disinvite her and DH's friend.