I'm missing what damage is being done here. A PC tries to use his conveniently placed sister to... Speed a meeting up a week? Is your campaign so time sensitive that the PCs meeting a noble a week sooner is going to derail it? And if it is, there are countless ways to add complications that can delay the request (the lady is out of town attending a relative's funeral, she is too distraught to take this meeting. I tried, sorry) or to drain other resources along with it (I got you a meeting, but the Lady doesn't take them this quickly without some flattery. I suggest an expensive bottle of wine and a small piece of jewelry to make her much more agreeable to listening to your concerns). I mean, has nobody ever heard of "yes, and..." Before?