should I do nothing?

Recently my DS and his wife went to visit my niece. My niece who is in her thirties has not had a good record with boyfriends in the past, and the last one was jealous and controlling. Her current boyfriend seems to be liked but I'm not sure how much we know about him. Anyway my DIL said after their visit that the new boyfriend seemed overly critical and bad tempered. My niece had told them that she couldn't discuss her new job openly with them because it threatened her bf's masculinity because he was in a lesser position. They didn't feel comfortable about him at all and now they are engaged. I would like to ask my sister-in-law if she thinks everything is OK but I would have to tell her about the private conversation I had with my son and his wife, and I wouldn't want it to get back to my niece that they had been discussing their visit with me. My DIL asked for my advice and I said I didn't see how she could do anything about it. I am still worried and don't know whether to keep mum about it or not? any advice appreciated.