Is the money a person gave to his niece considered a gift, and can the heirs demand their share of this money after his death?

The brief answer to the question of whether ablution (wudu') removes minor ritual impurity only or major ritual impurity as well is that ablution by itself does not remove major ritual impurity.

To simplify the matter, we can divide it into basic points:

Ablution (wudu') removes minor ritual impurity: This is by scholarly consensus. If a Muslim performs ablution, they have removed their minor ritual impurity, and thus it is permissible for them to pray, perform circumambulation (tawaf (definition)), and touch the Qur'an. Major ritual impurity requires a ritual bath (ghusl (definition)): Major ritual impurity cannot be removed by ablution alone; rather, it necessitates a ritual bath. Ablution is a part of the ritual bath: Although ablution does not remove major ritual impurity by itself, it is an essential part of the complete ritual bath. The ritual bath is not valid unless preceded or accompanied by ablution. The Prophet, peace and blessings be upon him, clarified the method of the complete ritual bath, stating that it begins with ablution. It was narrated by Aisha, may Allah be pleased with her, that: "When the Messenger of Allah, peace and blessings be upon him, took a bath from Janabah (definition) (major ritual impurity), he would start by washing his hands. Then he would perform ablution as he would for prayer. Then he would put his fingers in the water and run them through the roots of his hair. Then he would pour three handfuls of water over his head, and then pour water over his entire skin." Ruling on the prayer of one who sufficed with ablution instead of a ritual bath: One who has major ritual impurity and only performs ablution is not permitted to pray, and their prayer is invalid, unless they perform the complete ritual bath.

Conclusion: Ablution removes minor ritual impurity and is considered a part of the complete ritual bath for removing major ritual impurity, but by itself, it is not sufficient to remove major ritual impurity.