DollyMe is correct: what your sister wants done with her remains needs to be in her will. Her legally finalized will (witnessed and notarized depending on her state laws) will guarantee her wishes will be carried out.
What are her wishes? Has she made them known to anyone either verbally or in writing?
If she hasn’t told you do you feel comfortable asking her about it? Do you speak to her boyfriend? Can you ask him if he has spoken to her about her wishes?
If she hasn’t told you do you feel comfortable asking her about it? Do you speak to her boyfriend? Can you ask him if he has spoken to her about her wishes?