I'll be sure to have another conversation with my daughter before she leaves. She has had opportunities to talk about this in the past, and very directly said that she would rather not.
I've respected her wishes.
I see that as two big wins. You have a daughter who feels capable of setting and maintaining boundaries (I suspect I know where she learned that

), and you are respecting those and avoiding dragging her into a triangle situation.
My S18 returns from college in a little over a week. While I'm really glad S14 will no longer be facing the daily reminder of the separation alone, I am a bit nervous about how S18 will react to the new environment and how I will be tempted to react to his reaction.

I will definitely be watching for updates on how this is playing out for you.