Our father passed away when our eldest brother was 18 years old and our youngest was three months old. Our mother did not remarry. She took a vow to build a mosque with the value of a piece of land that was left to us by our father along with a lot of money. When we grew up, my mother did not have enough money to build a mosque as she had vowed because the value of the land is not enough to build a mosque now. Some of us need their shares in the inheritance, but we are not in dire need of it though. Can we take our shares in the inheritance and cancel the idea of building the mosque since our mother vowed to build it with the inheritance of her children who were minors back then, given that we have always accepted the vow as a fait accompli?
My second question is: my mother vowed to devote a significant part of her share in the inheritance to build that mosque, but now she cannot afford to build it. She is afraid of canceling her vow and intends to donate this money to some mosques? Is it lawful for her to take her share in the inheritance (and abstain from donating this money for building a mosque)? Should she donate it or a part of it?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Building mosques is one of the recommended righteous deeds and an ongoing charity for which a person continues to be rewarded even after death. The Prophet, sallallaahu ‘alayhi wa sallam, said: "Among the things whose reward will continue to reach a believer after his death are: Knowledge which he taught and spread; a righteous son whom he left behind; a copy of the Quran that he left as a legacy; a mosque that he built ..." [Ibn Maajah and Ibn Khuzaymah] Moreover, when a Muslim makes a vow to do an act of obedience to Allaah, the Exalted, he is obliged to fulfill it to the best of his ability. The Prophet, sallallaahu ‘alayhi wa sallam, said: "Whoever vowed to do an act of obedience to Allaah, the Exalted, must do it; and whoever vowed to disobey Allaah, the Exalted, must not disobey Him." [Al-Bukhari] In fact, we have not fully understood your question, but we understood that your mother vowed to build a mosque with the value of her late husband's piece of land, which is the inheritance of all the heirs and not hers alone. We also understood that her share in the land constitutes the largest part of her share in the inheritance. Based on that, such a vow is not valid or binding except for her share of the land. If her share is enough to build a mosque, then she is obliged to deliver on her vow and build the mosque. If it is not enough, then she should donate this money for building another mosque as this is what she is able to do. Al-Mubarakfoori said:

"The Prophet, sallallaahu ‘alayhi wa sallam, said: "There is no vow concerning something that a man does not possess." This means that when a person makes a vow about something that he does not possess, his vow is not valid or binding. Even if he possessed that object later on, he is not obliged to deliver on his vow and no expiation is due." [Tuhfat Al-Ahwathi Sharh Sunan At-Tirmithi]

Moreover, it is impermissible for her to cancel her vow or avail herself of the vowed money with which she had vowed to build the mosque. She should dispose of it as aforesaid, i.e. donate it for building a mosque.

Allaah Knows best.