Your older sister has a friend visiting. You've never been too happy with them any time they've been in the house, they always make you uncomfortable, at least with how much they flirt with you. They see no issue in letting the world know they like girls, but it means they flirt with you relentlessly whether you want it or not. But there's little you can do to stop them. You've always made it clear you aren't interested, but short of asking them to stop, or asking your sister to make them leave, neither of which is likely to happen, you can't do much but just take it.

At least that was what you thought, back when the worst they did was some innocent but unsolicited flirting. Suddenly they're going to escalate things more than anything you're ready for.

Maybe they happen to catch you walking out of the shower wearing just a towel, or in the morning wearing nothing but a long t-shirt. For whatever reason your sister isn't around to keep them under control, and there's little you can do to stop them pinning you against a wall, forcibly spreading your legs, and unleashing their expert fingers and tongues on you until you reluctantly squirt all over them.

Maybe instead your sister got into some trouble. Her friend bailed her out but now taking you as payment. You wouldn't get used infront of her right??

From there I'd love to discuss what happens. Maybe you learn to love what their fingers can do to you, finding any excuse to spend time alone with them, or maybe they don't care if you like it or not.