What is the definition of "ribat" (vaginal ligation), what are its harms, and is the mother held accountable for this action, given that she was unaware it was forbidden? And is the older sister sinful for refusing to undo the "ribat" despite knowing it is forbidden?

Binding (al-rabt) is a type of magic that affects the center in the brain controlling the genitals, thereby preventing erection and intercourse. It is one of the gravest sins and a form of disbelief (kufr). If your mother was unaware of its prohibition, then there is no sin upon her, and if she repents from it, the sin will be erased from her. Your sister must rid herself of this magic in obedience to Allah and out of filial piety towards her mother.