I'm feeling nosy. Do you really think you're all getting in bed together anyway if she's not interested in you? (aka that you should just roll with the excitement and let it happen) I only ask because if Lucy and Ethel are really into each other, and Ethel has sex with you out of "expectation" that it's supposed to be a hot sexy time, Ethel could go away feeling like it wasn't right for her. Then it's likely L&E would fizzle out if everyone's communication skills aren't functioning at their best.
I see the other post you made about the girl who had more feelings for you than you and Lucy expected and how it blew up. Just thought I'd suggest you are a bit more careful this time through. My experience was that I had a great week with a guy where there was lot of expectations so we did have sex, then the next time we were together I realized I wasn't interested in a relationship and that was an awkward transition. I didn't have a third party who wanted to still date him but I cant imagine that would be a comfortable situation so I do suggest talking about all the possibilities.