Is zakat obligatory on the inheritors' wealth starting from the date of the mother's death or from the date of taking possession of the money, and is zakat obligatory for the deceased mother's past years?

Zakat (definition) on an inheritance is not obligatory if it does not exceed the heirs' dwelling. Therefore, there is no zakat on the mother or her siblings regarding the house. Similarly, there is no zakat on the children or sisters regarding the mother's share of the house, unless each of them receives the value of their share, which reaches the nisab (definition), and a hawl (full lunar year) passes from the date of its receipt. The same applies to other possessions whose intrinsic value is not subject to zakat.

And Allah knows best.