Honestly, I'm not sure that the two are connected. Its a family trip. Do you want her, yes or no? What will you do in years going forward? For both of your girls? Will this be the last ever vacation you will pay for with dd2 as well? If you want her to go, and she wants to come, then it seems that you should work out something with her that works, not just for her now, but for her sister in future years as well. If you use the grades as the reason, what will you do if she buckles down next year and does well?
As for school, that's a separate issue. Are you co-signing her loans? Are you funding any of her education? Is anyone, or is she doing all that on her own? If you are funding any of her costs or signing loans, then you have a consequence for the grades. If you don't take her and she pulls her grades up, then what are you going to do?
I'm just now seeing the connection between the two - especially if you are not paying for her education.