Is it permissible for a mother to take her daughter's gold, which came to her as previous gifts, under the pretext that she has gotten married, promising to return it to her later, and is it permissible for her to favor one of her daughters in material gifts over another?

The question includes two parts:

First: The gifted gold became your property, and it is not permissible for your mother to take it and refuse to give it to you without your consent; for the Prophet, peace and blessings be upon him, said: "The wealth of a Muslim is not lawful except with his willing consent." Your marriage does not remove it from your ownership, and she has no right to keep it to give it to you at childbirth, nor is it permissible for her to gift it to you while she does not own it.

Second: The parents depositing a sum of money into your sister's bank account, if it was as a gift, then it is invalid; due to the necessity of equal treatment among children in gifting, and the impermissibility of favoring some over others without a need.

We advise you to be patient and seek help from Allah and send mediators from among people of virtue.