If my sister is 47 kg and 158 cm should she gain weight to get her period

Possibly: Your sister has a bmi of 18.8. While this falls into the "normal" range, it is, as you likely surmised, at the bottom of the normal range, the skinnier end, that is. Sometimes when women don't have enough fat, they might miss their periods. Her weight may be the answer, but there could be other reasons too. If she's worried she should get checked at the doctor.