They have acted exactly how they should of tbh Gra.

She is in a position of authority and can thus damage the Company further if she is found to be dishonest.

They have the right to suspend immediately on the spot and escort the person from the premises whilst a full investigation is carried out. This is not them saying she is guilty, this is their way of doing a through investigation without the concerned parties able to sabbotage any evidence and if they are a disgruntled employee it will stop them from doing more damage to the Company by abusing their position.

If your sister is guilty of doing it and she's going to a new job she can probably kiss goodbye to that im afraid