FWB interested in my sister

I’ve been FWB with a close friend of mine since August. My sister currently lives out of the area but is moving back in a month and will be living with me. I’ve been super excited as we are best friends. My FWB and sister are also really good friends and we hang out together all the time and all get along super well.

Two nights ago, my FWB and I were hanging out making dinner. And he said something about how I should watch out when she moves back because she might make a move on him (she doesn’t know we are hooking up, but I know she’s not interested in him). I played it off laughing, and asked him what he would do if she did? He laughed and said ‘I don’t know.. that might be a little too freaky for me. Also your dad would probably kill me..’ I just laughed, but immediately felt nauseous and wanted to leave but stayed.

Later after dinner, because I couldn’t stop thinking about it, I confronted him and asked if he was interested in my sister and that it was ok either way but that I needed to know because I wanted to stop the benefits if this was the case. His face flushed immediately and he said he wasn’t interested in her, only me. I told him that that would be crossing a major boundary for me and he agreed and said he never would.

I trust that he wouldn’t do anything with her because I asked, but the fact that it even crossed his mind is making me so upset. I guess what Im asking is whether I should continue my arrangement with him? I feel like when she moves back and we are all hanging out, I’m going to resent them both…