Thanks for your post and welcome to the community forums. Your niece would need to speak with her old provider to obtain a PAC number which allows numbers to be transferred between providers.There are few things to keep in mind:
If she is still in contract with the old provider, there may be a fee to cancel.
If the Virgin SIM is in your name, although the number is getting transferred over is your niece's, the virgin account will still be in your name as contracts are non-transferable.
If your niece has the PAC and would like to transfer the number, to the Virgin SIM, the data protection questions are based on your details as you are the account holder.