That is exactly how I feel. I used this analogy before when I once had a discussion about this:

Let's say I have a crush on Batman. My brother RPs as Batman online so that I can feel like I'm interacting with him, because it brings me comfort and joy to imagine myself with him. If I were to get sexually aroused, the arousal is coming from the idea of being with Batman. I'm not thinking of being with my brother. I'm thinking of the character, and that is arousing to me.

What distresses me so badly is that this can very easily be construed as incestuous. I can even see how it could be seen as that. Even though I, myself, and my brother, have never seen it as such; we, like you said, "separate the art from the artist." He's playing a part to bring me joy. And we would never do that in real life or if physical contact were involved. We would both be way too disgusted.

Outside of our online RPs - in real life - we actually have a completely normal sibling relationship. Neither of us is attracted to the other. Sometimes when hanging out in public, somebody will ask my brother if I'm his girlfriend. This grosses both of us out. We have the same normal, "ew, that's gross" reaction as any siblings would.

It's essentially my brother giving me a way to interact with my imaginary friend. It's stupid, it's cringey, it's weird, it's uncommon...but neither of us thought of it as incestuous.