Mahram Tehran BC was created in 1999.

Muslim woman can not leave the place unless she is with her mahram. Mahram is her husband, father, uncle, and any blood related.

It is not a serious crime. Don't mind

Traveling without a mahram is not permissible regardless of the reason of the travel. There must be a mahram present if the distance of travel is roughly 80 kilometers or more as this is considered travel, a day and night for one riding an animal, according to custom. So you cannot travel without a mahram, not to visit you sister or anyone else according to the statement of the prophet (sall-Allahu alaihi WA sallam), "the woman is not to travel except with a mahram" which was reported by Al-Bukhari and Muslim in their authentic collections of hadeeth. For more details, please refer to the below sites.

However, there is fatwa that Muslim woman can travel wothout a Mahram if she is in a good trustful accompany and secured from any evil deeds.

No. A woman's sister's husband is not Mahram of the woman. Mahram is a person who cant marry that woman in any circumstances. A woman's husband may divorce his wife and marry her sister. Therefore, he is not he rMahram. Father, father's father, real brother and his off-spring, real ucle, husband's father, real nephew, step brother, step son, are a few Mahrams of a woman who are forbidden to marry her. She may perform Hajj with them.