Praise be to Allah.
In the answer to question no. 82400 we have stated the ruing on conditional divorce and discussed the scholarly points of view on the matter in detail. There is also a warning against hastening to divorce or using divorce as an oath.
Your aunt should understand that it is not essential that she know about her divorce being conditional upon her doing something, and it is not essential that she hear her husband utter the divorce, but her husband should have told her what he had done. Not telling her what he said about her divorce being conditional upon her entering her sister’s house shows that he did not intend to stop her, rather he intended that the divorce should take place. If he had wanted to stop her, he would have made sure she heard what he said or he would have told her afterwards.
Whatever the case, it seems that he intended that the divorce should take place, and she became divorced when she entered her sister’s house, even if she did not know what he had said.
See also the answer to question no. 43481.
And Allaah knows best.