I'm assuming your sister has already removed the bobbin case and hook. It's amazing how many times that's where the problem is.
If the top is frozen the best solution is oil and patience. Other members have reported really good results with Tri-Flo synthetic oil for loosening up dried oil. Take off every cover and oil everything. It will take a while-sometimes days-for the oil to penetrate and free up the stuck parts. Oil, wiggle the machine and repeat. Eventually it will move.
Rodney