even the risk of oral herpes through sharing drinks is unlikely unless you are a small child/toddler.
we call them sti's for good reason - you have to be having sexual contact to transmit them.
grace
Gonorrhea or chlamydia is not transferrable through saliva, or drinking after someone else. Herpes is transmittable through this method if you have a type of oral herpes. If you are broke out around or in your mouth, it can be transferred onto the straw and you sister can get it. If you have genital herpes, it isn't.
Gonorrhea and Chlamydia cannot be transmitted in this method however HSV1 or oral herpes can be possible. If you had an outbreak of blisters or open sores in or around your mouth, it is transmissible.