In general, first person is tricky. It can either hinder or amplify the erotic effect you set out to achieve. For example. Supposed you're into the "young brother becomes strong and dominant" fantasy. Most people that find this setting erotic, will be attracted to the younger brother in the story. If you tell the story in first person, from the perspective of the young brother, the fantasy becomes a bit jarring, because the reader is forced to take the POV of their object of desire, instead of simply focusing on him. It would be easier, more relatable and more accessible to take the POV of the older brother, because he would serve as a perfect surrogate for the reader.
Third person narrative, by definition, is more inclusive. You are welcome to observe the fantasy in a more objective fashion, and you can decide who you want to focus on.
Of course, first person can be extremely effective if you manage to capture a fantasy that is relatable to your readers and are able to get them to identify with the narrator. There's a sense of immediacy and intimacy that would have been harder to achieve had you been employing a third person narrative.