What should the questioner do for her mother to forgive her for something she did not do, which made her not love her as much as her sisters, even though she treated her kindly in her last days?

Shortcoming in fulfilling the rights of parents is something that happens to everyone, due to the magnitude of this right. However, the occurrence of a shortcoming does not mean the occurrence of sin. If the prevailing state of the child is one of dutifulness towards their parents and beneficence towards them, then Allah will overlook any slip or outburst that may emanate from them towards their parents during moments of vexation and anger, in confirmation of His saying: ﴿Your Lord is most knowing of what is within yourselves. If you should be righteous, then indeed He is ever Forgiving to the oft-returning [to Him].﴾ (Al-Isra: 25). This feeling of guilt is expiated by sincere repentance, frequent seeking of forgiveness, persistent supplication for them and giving charity on their behalf, honoring their friends, and maintaining ties with their relatives.