He gave her a conditional divorce that would become effective if she spoke to her sister's husband, then she spoke to him without meaning to
Question

                I am married and my husband swore an oath that I would be divorced if I spoke to my sister's husband. Is this permissible? Please note that my sister's husband said to my sister one time: Give my salaam to your sister, and my sister told me about that and I returned the greeting to him. Has divorce taken place as a result? On one occasion I spoke to my sister's husband without meaning to. Has divorce taken place as a result?.
Praise be to Allah.
    Firstly: 

    If your husband's oath that you would be divorced if you 
    spoke to your sister's husband -- such as if he said: You are divorced if 
    you speak to him; or if he made divorce conditional upon that and said: If 
    you speak to your sister's husband then you are divorced -- if he wanted to 
    stop you speaking to him for a clear reason, such as if the man was not 
    proper in the way he spoke to you, or if he talks too much in a way that 
    annoys your husband, or he is afraid of some evil resulting from his 
    speaking to you, then in that case there is nothing wrong with him 
    preventing you, even if he (your husband) speaks to your sister. 

    It is well-known that you are a stranger or non-mahram to 
    your sister's husband, so it is not permissible for you to uncover anything 
    in front of him, nor is it permissible for you to shake hands with him, and 
    you must control your speaking with him so that there is no softening of the 
    voice or speaking too much unnecessarily, because you are a non-mahram like 
    any other. 

    Secondly: 

    This conditional divorce is subject to further discussion: 

    If the husband intended that divorce should take place if you 
    spoke to your sister's husband, then if you spoke to him, divorce has taken 
    place. 

    If he intended only to stop you doing it, and he did not 
    intend that divorce should take place, then this is an oath (and not a 
    divorce). If he wants to, he may let you off and offer expiation for his 
    oath. 

    This differentiation is the view favoured by Shaykh al-Islam 
    Ibn Taymiyah (may Allaah have mercy on him), and it was also stated in the 
    fatwas of a number of scholars. The majority of scholars, however, are of 
    the view that there is no such differentiation, and that if this talking 
    takes place, then divorce has occurred, regardless of the intention of the 
    one who uttered the conditional divorce. 

    Please see the answer to question number
    39941. 

    Thirdly: 

    If you spoke to him on the basis that you had forgotten your 
    husband's oath, then divorce does not take place according to the more 
    correct opinion, which is the view of the Shaafa’is and of Ahmad according 
    to one report. It was also the view favoured by Shaykh al-Islam Ibn 
    Taymiyah, and was regarded as correct by al-Mardaawi in al-Insaaf, 
    9/114. 

    Shaykh al-Islam Zakariya al-Ansaari said in 
    Asna’l-Mataalib (3/301): Similarly, the divorce does not take place if 
    he made it conditional upon the action of another person, such as a wife or 
    someone else, and he intended thereby to prevent that person from doing 
    something or urge him or her to do something, and that person would care 
    about this conditional divorce and would do something to make it happen, 
    because of friendship or the like, and he or she was aware of this 
    conditional divorce and did what he or she was not supposed to do out of 
    forgetfulness or ignorance or under compulsion. 

    Ibn Hajar al-Haythami said in al-Fataawa al-Fiqhiyyah 
    al-Kubra (4/178): If a person issues a conditional divorce or swears an 
    oath conditional upon him doing something himself, then he does that thing 
    because he forgot about that condition, or he remembered it but he was 
    forced to do it, or he did it by choice, unaware of the implications of the 
    condition, then he has not broken his oath, because of the report mentioned 
    above: “Allaah has forgiven my ummah for mistakes, what they forget and what 
    they are forced to do” – in other words, He will not punish them for any of 
    these three things. Similarly he has not broken his oath if he made it 
    conditional upon the action of another person who cares about this 
    condition, such that he would not go against it because of friendship, 
    embarrassment or chivalry, and he intended by means of that oath to prevent 
    him from doing something or urge him to do something, and the other person 
    was aware of the condition and he did it the action in question) out of 
    forgetfulness or ignorance or under compulsion.

    End quote. 

    See: Fataawa al-Shaykh Ibn Baaz (22/47). 

    Thirdly: 

    The fact that your sister conveyed to you her husband's 
    salaam and you returned his greeting is not regarded as speaking to your 
    sister's husband, so it does not affect you, but you should refer to your 
    husband to find out the limits of what he is allowing and not allowing, and 
    what he intended by his conditional divorce, because the intention specifies 
    the wording. So it may be that he intended to prevent you from speaking to 
    him at one time but not another, or in one situation but not another, or he 
    wanted to prevent too much talk between you, but not brief exchanges such as 
    saying salaam and the like. 

    What we advise you to do is to take care of your home and 
    family, and avoid talking to your sister's husband altogether, and keep away 
    from situations that lead to that, so long as your husband doesn't want you 
    to do that, in order to please your husband and to avoid divorce taking 
    place. At the same time you should think positively of your husband and look 
    for excuses for him. He may have a genuine reason for doing that. We also 
    advise you to observe hijab in front of your sisters’ husbands and your 
    husband’s brothers, because they are all non-mahrams for you. 

    We ask Allaah to increase you in guidance and wisdom. 

    And Allaah knows best.