My husband wants to sleep with my sister?!

My husband wants a threesome, he's been pushing for one for a long time now, which is okay we're all allowed to want things. At first I was not for it at all but eventually after enough pressure told him I may be open to one in the far FAR future.

The problem I do have with it and (it will not be happening) is who he wants it to be with..... My twin sister.

Who happens live with us. He said how could you not have those throughts coming home to two beautiful woman every day...

I've told him absolutely not and broughtup to him how might feel if I asked him to have a threesome with one of his siblings. This week he told me over text that he just wishes we could all be more open. Like what?!

Alwhile though it does worry me that he feels that way towards her I don't want to have to ask her to move out over it. She is my best friend and I don't feel like I'm able to talk to her about it because they are home alone together at times and I don't want her to feel uncomfortable. He is in most every other way a good man and treats me like gold, and I don't I don't think either of them would do anything together. I just don't really know what to do as he will not drop the subject, but it's caused a lot of anxiety for me lately and leaves me feeling like he doesn't respect my side of it. Help?