Please do tell your sister to stop behaving like a child and take the medication regularly. Tell her that not only her life but she will be putting the babies life to risk. She has some duty towards the baby that is going to come in this wonderful world.
Folic acid is required as it supplies the necessary iron to the mother and the baby which is necessary for the baby for normal birth.

What she can do is go for alternative foods that contain folic acid. But frankly it would be better if she paid heed to the doctors advice. She can go for foods like leafy green vegetables such as spinach and turnip greens, dry beans and peas, fortified cereals and grain products, and some fruits and vegetables are rich food sources of folate. Some breakfast cereals also contain folic acid and good iron levels.