Is it okay that I sniffed my sister's spent tampons
No it's not! Why would you do something disgusting like this?
I feel sick when I smell my sister's used tampons.
I'm not surprised you feel sick. This is disgusting and very unhygienic. You can catch a disease or bacterial infection from mishandling trash.
I wanted to see what my sister was using it for. I'm curious about this.
There are better ways to find out what the tampon is for than sniffing it. I'm sure your parents would answer the question for you. In the future leave the trash in the trash.