All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It is permissible for your sister to delay her monthly period in order to perform all the rituals of Hajj (Pilgrimage), especially the Tawaaf (Circumambulation around the Ka’bah) of Hajj.
This permission applies in the following cases:
1 - If her period will prevent her from being accompanied by her Mahram, thus preventing her from travel.
2- If she will suffer financial or material loss resulting from having to delay or cancel her travel.
3 - If the occurrence of her period could cause legal (Visa, immigration, travel) problems during the Hajj period. Moreover, Muslim scholars set a condition that using this medicine should not be something that could cause any harm to the woman.
For more benefit on using pills to delay menses, please refer to Fataawa 83526, 85015, and 28479.
Allah knows best.