Is Zakat obligatory on the money collected for the sister from selling the share of the mother and siblings from their deceased father's land, which has reached the Nisab and a complete year has passed over it, with the intention of helping her get married, knowing that the sister has reached the age of maturity? And what should be done if Zakat was obligatory for the past year?

If your sister took possession of the money, then it is her property and she is obligated to pay zakat on it if it reaches the nisab (minimum threshold) and a hawl (full lunar year) passes over it. She must pay the zakat for the past years. However, if she did not take possession of it, then the money still belongs to you, and you have the right to revoke the gift. Zakat is obligatory on each of you for your share if it reaches the nisab when combined with what you already possess.