I've done Sister/Sister but it was years apart and while I was in the military. I came home on leave and one Sister saw me in uniform and soaked her panties and we hooked up a couple of times after Summer activities.
About 5 years later, the Sister that I'd always had the biggest crush on when I was a teen had gotten divorced and I happened to be home in time for her and another brother's birthday party. There were 14 of them so a few had the same or in the same couple of week span, etc.
We had drinks and she had a twinkle in her eye when I told her that I used to beat off to her at night sometimes.
"Awww, how come you didn't ever tell me that back then?"
I was 16 and runnin' the streets witcho youngest brother...all you were gonna do was pat me on the head and tell me that was cute and you're flattered.
"Yeah, that's true."
I gave her the line from Switch...But I'm a MAN now baby, a GROWN MAN, and I've come a looong way!
We kicked it for the whole month that I was home on Leave. It helped that the other sister wasn't in town while I was there, she probably would've tugged on her sister's sleeve.
My first Sister/Sister combo was as a younger teen down South and lead to a Cousin/Cousin encounter cos of Chuuch, you know. The first Sister was the first time I'd eaten pussy too. It was so pretty, fresh, and fragrant. I didn't know what pheromones were back then though.