Is my sister's seclusion with her husband, who has not consummated the marriage but was sitting and talking with her, considered as consummation? And is she entitled to the full mahr? And must she observe an 'iddah if she is divorced?

Divorcing a wife before consummation or valid seclusion (khalwa sahiha) is an irrevocable divorce (طلاق بائن), meaning the husband does not have the right to take her back except with a new contract. There is no waiting period ('idda) for her in such a case, based on the Almighty’s saying: {When you marry believing women and then divorce them before you have touched them, then there is no 'idda (waiting period) for you to count concerning them}. Valid seclusion (khalwa sahiha) is that in which intercourse can ordinarily occur, as opposed to merely being alone together without security from others entering. Therefore, if the matter was limited to the woman sitting with her husband in a place where the entry of others was not secured, or her going with him to prepare the car, this is not considered valid seclusion. If he divorces her, there is no 'idda for her, and he does not have the right to take her back except with a new contract. She is entitled to half of the stipulated Mahr, unless one of them waives their half for the other. If a woman asks her husband for a divorce without him having harmed her, he has the right to refuse to divorce her until she forfeits her right to the Mahr, or gives him what they agree upon.