In my case (and I suspect many other people's), the incest fetish is about the concept, without going so far as projecting towards my own relatives.
Ie. I role play the MC, and I desire HIS mom and sisters, not mine.
It draws only conceptually on my own experience since I also have a mom and I've lived all my young life in the same home as her so all these domestic situations are familiar, but I never desired my own mom in those situations. I don't imagine her when playing, at all.
I also never use actual names in my family for characters - that would be crossing a line, especially since I still see them regularly. In my opinion that would be mentally very unhealthy.
Anecdotically, I also never use my own name for the MC. I live someone else's live.