In the 70s "twilight sleep" was usually a morphine-type drug or it could even be somethng inhaled like nitrous oxice. Fortunately most of the drugs that have been used to induce the sedation were safe for us. Many women felt robbed because it was imposed on them in the last few minutes of the birth and they could have handled it just fine without.
Be sure your family member tells the anesthesiologist at the pre-op appointment or phone call about the family history of myotonia, and then tell them again before surgery since they are often different doctors and the one overseeing the surgery may not have read the notes.