Is the money that the mother gave to her son before her death, and did not mention that it was a loan, considered part of her estate?

The amount given by the mother to her son, without explicitly stating it as a gift (hiba) or without any indication thereof, is not considered a gift. It must be returned to the inheritance to be distributed among the heirs according to Islamic law.

However, if the giving was accompanied by something indicating the intention of a gift, then what matters is the time when the amount was given: 1. If it was during the mother's death-sickness (maradh al-mawt), the gift is considered a bequest (wasiyya), which is impermissible for an heir. The amount must be returned to the inheritance. 2. If it was when she was in good health or during a non-fatal illness, then the gift is complete, and the money belongs to him.

If there are other children and justice was not achieved in the gift to that son without a legitimate reason: The majority of jurists hold that an unjust gift becomes effective after the death of the giver and is not returned. Shaykh al-Islam Ibn Taymiyyah is of the opinion that an unjust gift must be returned even after the death of the giver. The favored son must return the excess so that the money is distributed justly.

All of this applies if there was no legitimate reason for preferring that son. However, if there was a legitimate reason, such as the poverty or illness of the gifted son compared to his siblings, then there is no harm in him owning the amount if it was given to him when the mother was not in her death-sickness and with the intention of a gift.