Is the marriage still valid after the husband issued the first divorce to his wife, and then the second and third divorces at the same time over the phone, and after she returned and requested to go back to her husband, and how can the spouses return to each other? And is it permissible for them to perform a tahlil marriage?

It is not permissible to combine a woman with her sister's daughter [in marriage], because there is a consensus on its prohibition, based on the saying of the Messenger of Allah, peace and blessings be upon him: "A woman and her paternal aunt shall not be combined [in marriage], nor shall a woman and her maternal aunt." Combining relatives leads to animosity and the severing of kinship ties. He must separate from the sister's daughter immediately. This marriage is invalidated by consensus on its prohibition, and the woman must observe an iddah (waiting period) of three menstrual cycles. It is obligatory to avoid her because she is a stranger [to him, i.e., not his wife].