I think you did the right thing in talking to your fiance first. That's the first person you should speak with, because if I was engaged and my fiance spoke to my family instead of me first, I'd feel hurt. I'm the person he should first entrust with this information.

If your fiance had said to stay out of it, you would have to decide which you considered the right thing to do. If this sister is being harmed by having her meds withheld, she needs someone to speak up for her, because it doesn't sound like she has anyone to do so. It's like a child being abused has no one if the persons who know of the harm being done say nothing.

But you don't know who, if anyone, is doing anything wrong. As myself said, there could be a logical explanation. I think it's best if your fiance, with or without you, asked the parents face-to-face, and expressed both of your concerns on the matter. It is possible the sister is faking the taking of her meds; if the parents don't know how to check properly, she could hide the meds in various ways. She could even regurgitate them.

The best thing to do is ask, and perhaps watch for other signs in the future, rather than call a service to step in. You should look for more evidence first.

I don't know about doctors for physical illness and seizures and such, but for persons on psych meds, at least in NJ, the doctors or maybe even the law, expects patients to get blood tested periodically, to make sure you're taking the right amount of meds and to see how they're being tolerated. If she's not getting enough medication, a doctor might take notice if this kind of testing is being done.