How can I get fried prawns to be juicy?
You are overcooking them. When the shrimp turn pink at the end of your second step, they are done. When you add your remaining ingredients and continue to cook them, you are overcooking the shrimp.

I'd suggest cooking the shrimp until they are pink, then setting them aside. Cook your remaining ingredients in the pan until done, and then toss with the shrimp and serve.

Regarding the oil, it shouldn't make a difference that you are using sunflower oil. Regarding the frozen prawns, unless your mom was purchasing them direct from an oceanside market, she was likely buying once-frozen prawns too. Most prawns found in supermarkets are flash frozen at the source, and are only thawed and put on ice at the supermarket.