Being asked to do something not comfortable with

So my sister has asked me to go to the doctors, and get an sti check under her name. She needs this for her work but wouldn’t be approved if she went for the test herself.

She told me about how they test it and I’m really not comfortable doing it but she is heavily relying on me to do this for her.

What should I do?

Also, if I don’t do it, what should I say to her?