She is asking her husband to divorce her co-wife
Question

                I left my country to live in an environment more condusive to learning about Islam, and Alhamdulillah, Allah provided me with a wonderful family who looked after me, JazahumAllahu kheyran.  The father of this family took the responsibility of walayah (my family is not Muslim) - in looking for an appropriate husband for me, making arrangements for my accomodations, provision, and Islamic education.  As we began to go through potential husbands, he became aware that he had most of the qualities I was looking for.  He first discussed it with his wife, and then several months later opened the subject with me.  

However, his first wife made it clear to me that she didn't want the marriage to take place. I requested another brother to take the role of wali for me, and on his advice, and the advice of other learned people who know him and his family, and after much istikhara, the marriage took place.  My question is two parts - Now, the first wife is claiming that my marriage to her husband is ungratefulness for all of the kindness she had previously shown me, and that I oppressed her by responding to her husband's proposal without consulting her, and by accepting the marriage with the knowledge that she wasn't happy with it.  

1. Is her claim valid?  Have I oppressed her, and did our former friendship create a relationship between us that should have prevented me from marrying her husband?

Since then, she has insisted on divorce if he continues with me.  As they have seven children, and I have none, the mafsadah in divorcing her was greater than the mafsadah in divorcing me.  And as such, she forced him to divorce me even though I was giving up much of my rights to his time and agreed to her stipulations that their children should not know of the matter.  

2. Is it within the rights of the first wife to force her husband to divorce the second wife .
Praise be to Allah.
    Firstly: 

    It is not permissible for a woman to ask her husband to 
    divorce his second wife: 

    It was narrated that Abu Hurayrah (may Allaah be please with 
    him) said: The Prophet  (peace and blessings of Allaah be upon him) 
    said: “It is not permissible for a woman to demand her sister’s divorce so 
    that she may take her place and get married; she cannot have more than what 
    is decreed for her.”

    (Narrated by al-Bukhaari, 5144; Muslim, 1413). 

    According to another version, the Prophet  (peace and 
    blessings of Allaah be upon him) forbade women to stipulate (in the marriage 
    contract) that their sister be divorced. (Narrated by al-Bukhaari, 2577). 
    Al-Bukhaari included this hadeeth in a chapter entitled “Marriage conditions 
    which are not permissible.”

     (a)  Ibn al-Qayyim (may Allaah have mercy on him) said: the 
    ruling of the Prophet  (peace and blessings of Allaah be upon him) 
    indicates that it is haraam for a woman to stipulate that her sister be 
    divorced, and that the husband is not obliged to fulfil this condition.

    (Zaad al-Ma’aad, 5/107). 

    (b)  Al-Haafiz Ibn Hajar said: The apparent meaning of the 
    phrase “it is not permitted” is that this is forbidden. This is to be 
    understood as being the rule so long as there is no reason to allow that, 
    such as there being some suspicion concerning the woman which implies that 
    she should not remain married to the husband. The woman (the new wife) may 
    suggest that to the husband by way of advice only or so as to avoid harm 
    that either party may cause to the other.

     Ibn Battaal said: Saying that it is not permitted clearly 
    means that it is forbidden, but that does not necessarily mean that the 
    marriage is annulled, rather it is to emphasize that the woman (the new 
    wife) should not demand that he divorce the other wife. She should be 
    content with that which Allaah has decreed for her.

    (al-Fath, 9/274) 

    (c)Al-Nawawi said: the meaning of 
    this hadeeth is that it is not permitted for a non-mahram woman to ask the 
    man to divorce his wife and to marry her so that she becomes the only one on 
    whom he spends, whom treats kindly, has intercourse with, etc, as the 
    divorced woman used to be. 

    Sharh Muslim, 9/193 

    Based on the above, it is not permissible for the first wife 
    to ask her husband to divorce you, so you should not pay any attention to 
    what she says. Note that this is a case of the jealousy that exists in all 
    women; indeed, jealousy existed even in the best of women, namely the wives 
    of the Prophet  (peace and blessings of Allaah be upon him), the Mothers 
    of the Believers. Jealousy is something natural. 

    Secondly: 

    The fact that she had treated you kindly is something for 
    which she will be rewarded, but that does not allow her to ask her husband 
    to divorce you. So be patient and ignore her demand, and treat her kindly as 
    much as you can. The first wife should realize that she will not have 
    anything other than that which is decreed for her, as it says at the end of 
    the hadeeth quoted above from al-Bukhaari.

     Al-Haafiz Ibn Hajar said: 

    Hence he ended by saying, “she cannot have more than what is 
    decreed for her,” to show that even if she asks for that or insists on it 
    and stipulates it as a condition, nothing will happen except that which is 
    decreed by Allaah.

    Al-Fath, 
    9/275

     And Allaah knows best.