I can see your dilemma NL. I think, if this relationship between dad and son had started at a time when the son was still under age, you should draw a line.
However, if it didn't start until the son was 20+ and if both dad and his son are fine with what they're doing ... why then should you have "moral" regrets just because "in the eyes of the law" what they're doing is illegal. Another thing is, that you can never really know when this relationship started ... just because the both say it didn't start before the son was 20+ it's not necessarily true. Maybe it started at an earlier point ... only the dad and his son know for sure.
Also, have you been considering, that the son could be exaggerating the number of times he's been bred by his dad? Maybe the son thinks you're turned on by this kind of relationship, and for that reason he exaggerates the number of fucks.
I guess, that if both of them are happy with what they're doing ... and from what you tell it seems like they are ... then there's no reason for you to feel bad about going ahead with your plan of getting tag teamed by both of them.
Whatever you decide, I hope you will let us know your decision.