by peabo85
I like original approaches and yours is definitely out of the ordinary. I find myself wishing the younger sister would consent, but it's actually more original if she doesn't. More realistic too.
The power imbalance is good, keep that. Unless you want to develop deep characters and relationships, you need to go the other way - more sex. Make the younger sister struggle, but she will be getting it and giving it anyway. You just need to make it happen.
If you want to add nuance, make the older sister hesitate just a bit. On some level, she realizes how fucked up this is, giving the younger sister a glimmer of hope. But they will still make the plunge. The younger sister's turmoil could be enhanced by the conflict of how good it feels, even if she doesn't want it to happen, especially not at the hands of her sister. I don't think it should be enough to "convert" her though. It's still fucked up and she should always keep struggling.