My point of view - I would stay out of it.

1. You did not witness the person putting the pills into the toilet. It could have been the parents, it could have been the sister

2. it could have been an honest dropped the pills on the floor so the person put them into the toilet right away so that the unsanitary ones would not end up being taken. I myself have flushed dropped pills of my own.

3. The pills could have also been expired and when cleaning out the medications I throw expired meds down the toilet.

4. Without actually witnessing the act you have no idea who or why those pills were there. So charging in like a bull accusing, confronting and bringing the problem up will result in possibly starting a needless fight where there is no cause for it which could possibly trigger the sister into having stress related seizures.

5. you flushed the evidence so even if you confronted it is your word against those you are confronting possibly for no reason since you do not know the whole thing about the situation surrounding the pills.

6. the doctor may have prescribed a different medication and the person put the old prescription into the toilet.

7. your acting suspicious of your future inlaws will also cause problems not only with the sister and inlaws but also your fiance. The result of the fight over the pills can and may very well result in the engagement being called off and you and fiance going your separate ways.

There are so many what if and unanswered questions in this situation.

If I was ingaged to someone I would love that person enough to not want to cause problems for him and his family by starting a fight about something that did not involve me in the least.

I understand that you care for the sister but starting a fight about something you were not involved in is NOT the way to show it especially when you do not know anything but the fact that you saw some pills in a toilet and instead of asking about them when you found them you flushed them, tried to match what you remember to pictures on websites - there are many pills out there that resemble each other right down to certain markings. And now that the situation is done and over with you want to do something about it. The time to bring this up was when you found them not hours, days and weeks later. depending on how long it has bee. Obviously the sister has not had a seizure or ended up in the hospital or other bad scenerios from the situation other wise you would have put that into your first post in this thread.

So its time to drop it and stay out of things that you are not personally involved in for the sake of your engagement and relationship with the sister and inlaws.