His sister gave up her share of the inheritance then she regretted it
Question

                My sister gave up her share of a store to me after our parents died, but now she regrets it and she wants to change her mind. Does she have the right to do that?.
Praise be to Allah.The way in which we address 
    this problem will be based on an attitude of kindness, generosity and 
    chivalry, and on the basis of what we would like to think of you, that you 
    are generous, decent and of noble character, because our great religion 
    teaches us to be kind, uphold ties of kinship and respond to good treatment 
    in kind, because she is your sister who grew up with you and you share ties 
    of kinship with her, and she has great rights over you with regard to 
    looking after her, managing her affairs and striving to make her happy. So 
    it is better for you to be kind to her and give back what she gave up for 
    you, and to respond to her first act of kindness with one that is even 
    greater, and to let her off so that Allaah will let you off on the Day of 
    Resurrection. 
It was narrated that Abu 
    Hurayrah said: The Messenger of Allaah  (peace and blessings of Allaah 
    be upon him) said: “Whoever agrees with a Muslim to cancel a transaction, 
    Allaah will forgive his sins on the Day of Resurrection.” Narrated by Abu 
    Dawood (3460) and classed as saheeh by al-Albaani in Saheeh Abi Dawood. 
According to a version 
    narrated in al-Sunan al-Kubra by al-Bayhaqi: “Whoever agrees with one 
    who regrets it to cancel the transaction, Allaah will forgive his sins on 
    the Day of Resurrection.” 
Ibn ‘Abd al-Salaam said: 
    Agreeing to cancel a transaction for someone who regrets it is an act of 
    kindness that is enjoined in the Qur’aan. End quote, quoting from 
    al-Mannaawi in Fayd al-Qadeer (6/79). 
Rather, we say to you: It 
    would have been better for you to return to your sister what she had given 
    to you as soon as you realized that she regretted it, and you should not 
    have forced her to say that to you in words, so that she would not feel 
    embarrassed or awkward. 
Ma’mar – who was one of the 
    best of the Taabi’een – said: 
One of the worst acts of 
    kindness is when you force a person to ask (for what he needs) when he feels 
    shy before you, so your act of kindness will not make up for the pain of the 
    shyness that he went through. It would have been more appropriate for you to 
    find out about what your brother needed and send him what he needs, and not 
    force him to ask. End quote. 
This is how it should be 
    among siblings, for their joy is one and their grief is one. 
Always remember the great 
    reward that you will have with Allaah if you do what your sister wants and 
    give back that which she regrets giving, and relieve her distress, and 
    continue to be kind to her and uphold ties of kinship with her, especially 
    since she regrets giving it up because she is in difficulty and is facing 
    poverty. In that case you should offer her help and give her that which will 
    relieve her hardship, and reward her for her initial giving up of her share 
    to you. Allaah says (interpretation of the meaning): “Is there any reward 
    for good other than good?” [al-Rahmaan 55:60].  
The reports quoted above 
    were quoted from ‘Uluw al-Hammah by al-Sayyid al-‘Afaani (2/612-651, 
    5/288-302). 
But if she initially gave 
    it up because of shyness due to the fear of blame in a society that 
    mistreats women with regard to inheritance, or for fear that her brothers 
    might sever ties with her if they thought that she was insisting on her 
    rights, then in that case it is haraam for you to consume that wealth, and 
    it must be given back to her. The fuqaha’ have stated that whatever is taken 
    due to shyness is haraam wealth. 
It says in al-Mawsoo’ah 
    al-Fiqhiyyah (18/263): 
The Shaafa’is and Hanbalis 
    state that if wealth is taken due to shyness, such as if he asks someone 
    else for wealth in front of people and he gives it to him due only to 
    shyness, or he gives it to him out of shyness, and the recipient knows that 
    the giver only gave it to him out of shyness, it does not become his 
    property and it is not permissible for him to dispose of it, even if the 
    recipient did not ask for it. What matters here is the knowledge that the 
    one who gave his wealth gave it out of shyness, not out of generosity and 
    not for something in return.  
Hence if he joins people 
    whilst they are eating, and they ask him to eat with them, but he knows that 
    it was only because they felt shy, it is not permissible for him to eat 
    their food. End quote. 
To sum up: You should 
    return to your sister that which she gave up to you, so as to preserve love 
    between you and uphold the ties of kinship. 
We ask Allaah to compensate 
    you with something better. 
And Allaah knows best.