I hope someone can suggest stories along these lines.
To me a story about incest between siblings should crackle with tension. Ambiguous signals of possible interest: what did he mean by that? Will she scream at me, hate me forever and tell mom and dad? Tiny steps to test the waters: an unusual kiss on the cheek, a hug that's a wee bit longer than normal, little touches increasing in frequency that have not been normal but are completely innocent if the sibling is called on it. At the same time we should be in the heads of the siblings watching the sexual tension grow to almost unbearable.
I don't know if anyone here has read it, but years ago there was a story on Lit, "Incest by the Book" where the brother started with a clothed massage of his sister and convinced her baby step by baby step that this wasn't incest according to the dictionary. I loved that story, but unfortunately it was removed before I copied it.
The story doesn't have to be incest but I haven't found much outside that category. No luck at all with First Time category.
Any recommendations would be appreciated.