Pushing I****t Continued

A while back I wrote about my sister asking me to talk to her son and suggest he fuck his sister. My sister and I had sex when we were kids. Today I learned my sister is now divorced with full custody of her 2 kids. She talked to both of them about fucking and they have been trying things out. I went over to my sister's for Thanksgiving. When I got there my sister met me at the door and let me in. She told me her kids were in their rooms. I said I'd go in and say hi. She told me to be sure to knock first. I went to my niece's room, I could hear her bed squeeking and her moaning. The door was partialy open I looked in and saw my neice and nephew fucking. I went back to my sister and told her what i saw. She told me to knock! She said she had "the talk" with them and told them to try things with each other like we did. She went over to the room and told them to finish up and take a shower before dinner. We had a normal ish meal and even watched some football. After about an hour or 2 my neice and nephew disappeared. After a few minutes it was clear what was happening as you could hear some moaning and the occasional bed squeek. I asked my sister how long this has been going on? I was told it started 2 weeks ago after she talked to them and gave her ok. She said the only problem was that since she said she was ok with it they didn't always do it in private and they have left their doors open several times. I didn't check on them or try to get a peek. As far as I'm concerned I already know too much their sex life and don't want to know any more.