Walaikumussalam
Really it is disturbing situation and difficult to handle when someone is not ready to understand and giving threat of suicide.
Discus the matter with your mother's family . May be they can assist you and mother will listen their word. And explain to mother that it is haram and talk about Azab and punishment and impact of her act on children and family
May Allah guide her and give her taufeeq to refrain herself from Haram
Wassalam