A sister, in her full consciousness and with her own willingness, gave up her share of inheritance in favor of her brother. In the years following that, the brother asked his sister multiple times whether she wanted her share back, but she always insisted that she did not want it. The question is whether the share of inheritance that the sister had forsaken comes in the category of Hibah (gift).
Secondly, if now, 22 years later, as the property prices have greatly increased, she asks her share back, will it be regarded as taking back a Hibah, and will the hadith which says that taking back a Hibah is like a dog eating his vomit apply here? The brother fully realizes that he is bound to return it. Thank you.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger. 

If it is proven that this woman gave up her share of inheritance in favor of her brother and he took possession of it and became able to dispose of it freely as its legal owner, then it is considered a valid Hibah (gift), and it is impermissible for her to cancel the gift or ask her brother to give it back to her. The Prophet, sallallahu ‘alayhi wa sallam, said, “It is not lawful for a man to give a gift and then take it back, except a father regarding what he gives his child.” [Ahmad, Abu Daawood, An-Nasaa’i, At-Tirmithi and Ibn Maajah - At-Tirmithi, Al-Haakim, and Ibn Hibbaan graded it Saheeh (sound)]

Moreover, Ibn ‘Abbaas, may Allah be pleased with him, reported that the Prophet, sallallahu ‘alayhi wa sallam, said, “The one who takes back his gift (which he has already given) is like a dog that vomits and then eats its vomit.” [Al-Bukhaari and Muslim]

Another version of the Hadeeth reads, “The bad example is not for us. He who takes back his gift is like a dog that swallows its vomit.” [Al-Bukhaari]

If this woman denied giving up her share in favor of her brother and he does not bring forth evidence indicating that she had given it up, she is entitled to claim her share of the inheritance. If there is a dispute, then the matter should be referred to the Shariah court, if any exist in your country, or you can consult with reliable scholars directly so that all parties can be heard before reaching a verdict. The Prophet, sallallahu ‘alayhi wa sallam, said to ‘Ali, may Allah be pleased with him, “When two men come to you seeking judgment, do not judge for the first until you have heard the statement of the other. If you do so, then it will become clear to you what verdict to issue.” [Ahmad and Abu Daawoodl - Al-Albaani graded it Hasan (good)]

Allah knows best.