This is probably not going to be a popular response, but here goes anyway.
She may be really afraid of the dark. But you have to help her get to the bottom of why. She might not know why, in which case, you might need to take her to see someone about it.
In the meantime, tell her you want to understand, you really do, but it's not fair to make her little sister get up and go with her or turn lights on for her. If she is afraid there is something dangerous in the dark, is she sending her little sister so it can get her first? That's not very nice if that's what she's thinking.
I mentioned a few days ago in a response that my daughter "developed" a fear of balloons, of all things. She was afraid they would pop and be loud and hurt her ears. She is 10 years older than her little brother and she got to the point where she didn't want anyone else having balloons, including her little brother being able to have balloons at his own birthday party. I told her it wasn't fair because HE wanted them, it was HIS party, and SHE needed to knock it off. I'd had enough of her throwing fits over balloons.
She informed me she wouldn't go into Safeway one day because they had mylar balloons by the bakery and cakes when you walk in.
I was like, "Seriously? We can't go to the grocery store anymore?"
Mylar balloons don't even pop.
I might have been a little insensitive, but it was becoming a real control issue for her and when she informed me no balloons for her little brother, I had had it.
She didn't have the right to make the decision what he could and couldn't have.
It ended that day.
I was done, she knew it.
She's 24 and guess who personally delivers balloon bouquets for people for birthdays or a new baby or if they're sick.
My daughter.
She can't have a celebration without balloons, for heaven's sakes.
Go figure.
I just thought, at the time, that she was either going to get over it, and stop disrupting everything and everybody else, or she was going into therapy over it. Over balloons.
I had tried everything else. There was only two ways it could go.
I hope you get some good responses. If you have tried everything else, all I can say is to try not buying into it. At least for a couple of nights.
No more enlisting her little sister.
That's just my opinion and I don't mean it to sound harsh or uncaring.
I'm not that kind of mom at all, believe me.
Maybe just try a different tactic.
Best wishes.