Pushing I****t

My sister and I had sex growing up. We lost out virginities to each other I was 12 and she was about 14. We had sex pretty regularly after that. We were latch key kids and our parents worked late often. We stopped when she went to college.
Fast forward to present day. My sister is married and has a son and daughter. She asked me to talk to her son about sex. When I asked why his dad can't do it I was surprised. She wanted me to talk to him about sex and suggest he fuck his sister. She was going to have a similar conversation with her daughter. She thinks it's safer for them to have sex with each other than dating and possibly having multiple partners. I know she and I did this but no one suggested it, as far as I know. I think it's best if I leave it alone, let the kids do what they are going to do, and hope my sister gets this idea out of her head.