I hadn't been with DP long, and MIL for some bizarre reason, pretty much the first time we were alone, decided to tell me at length, how horrible DPs DF had been during their marriage. He's dead, and they were divorced years previously, so literally no reason at all to tell me.
Then she said 'you mustn't tell DP'.
I wrestled with it for a while, and then asked a friend who I really trusted to give good advice Wtf should I do? She said as PP have - not my secret to keep, my loyalty is to DP not MIL, she was very unfair to tell me and I should tell DP. I took her advice, she was absolutely right. DP was unhappy about the content, but not with me.
As his parents know, I'd tell them she told you, that it's put you in an awful position, and that as your loyalty is to DP you cannot keep this secret from him and think it would be much better and less painful coming from them than from you.
To be clear, GM has contact with the bio grandad's relations, DP knows them, but no one has ever told him who they actually are? That's so bizarre!