NC for this.
DD = DDad
DS = DSibling
My DM died a few years ago. DPs were happy together when she died but before this had a few problems. I think they both had affairs at one point or another. A number of years before DM died my DD had an affair which we all found out about. My DD has now started seeing the OW again. I am not thrilled about it but I know my DM was no angel and that my DPs' relationship was problematic though they loved one another a lot. I don't think the OW was in the picture for a long time before my DM died.
My DS is wonderful but volatile and a bit immature at times. I know DS would go absolutely off the charts angry if DS heard about this. I said this to my DDad when he told me about new relationship with OW and we both said we shouldn't tell DS unless the relationship with OW becomes serious or they decide to move in together or whatever. I don't feel great about keeping this from DS but DS would not handle it well. DS and DDad are close and DS relies heavily on him financially (we are talking gifts in the tens of thousands of pounds over the past few years). It would be awful if they fell out, for the whole family (I have one other DS). I would struggle if we got into a position where I felt I had to take sides as we are a very close family.