Your plant is not a true Jasmine. It is a Stephanotis floribunda, but it has equally fragrant flowers. It does best right in an east or west facing window as it needs several hours of direct sun each day and lots of very bright indirect sun the rest of the day. Not sure if the location in the photos is where you keep it, but it is not sunny enough there.
Water it when the top half-inch of soil is dry. Disturb the roots as little as possible. Keep it warm year round.
You might check with your sister and see what she is dong differently.