I did tell her I was annoyed by her not coming to me first but she told me she wanted to see if he would do it as if he said no it wouldn't of be worth asking me.

We have been together a few years and we are close to "officially" moving in together.

She explained that there wasn't really ever any emotional feelings between them it was just sex. They would wait until their parents left and that's when they would do it. It was more like a habit for them.

I think this is a kink of hers and the incest side of things does complicate the breakdown of it all. Bit she explained that she would be jealous of another girl touching me and wouldn't trust any other guy to not complicate things. She fully trusts her brother to be their purely to satisfy her kink and nothing more.

I'm rather vanilla when it comes to sex so this is all just a lot to take in.