One of the wives is seeking the help of unjust laws to cause harm to the other wife; what is the ruling?

Firstly: 

Undoubtedly if one of the wives, whether it is the first wife or otherwise,
strives to bring about the divorce of the other wife, that is haraam and is
striving for the sake of falsehood, and for that which angers the Most
Merciful and pleases the Shaytaan. This comes under the heading of
covetousness and selfishness. Allah, may He be glorified and exalted, says
(interpretation of the meaning): “And
whosoever is saved from his own covetousness, such are they who will be the
successful” [al-Hashr 59:9].
This indicates that the one who is motivated by covetousness and pursues his
whims and desires is a loser. 

It
also comes under the heading of thinking negatively of Allah, may He be
glorified and exalted, and of weak faith in His will and decree, for all
provision is in the hand of Allah, including the love of a husband for his
wife, the time he spends with her, and his interest in her. All of that is
included in the provision that is granted by Allah, and that which is with
Allah cannot be attained by disobeying Him. 

Al-Bukhaari (5152) and Muslim (1408) narrated from Abu Hurayrah (may Allah
be pleased with him) that the Prophet (blessings and peace of Allah be upon
him) said: “It is not permissible for a woman to ask for her sister to be
divorced so as to deprive her of what is rightfully hers and take it for
herself; rather she will have what has been decreed for her.” 

Ibn ‘Abd al-Barr (may Allah have mercy on him) said:  

From this report we understand that it is not appropriate for a woman to ask
her husband to divorce her co-wife so that she can have him to herself;
rather she will have what has already been decreed for her, and the divorce
of her co-wife will not detract anything from that which has been decreed
for her, or add to it. 

Al-Akhfash said: It is as if he meant that she seeks to deprive her of her
husband’s goodness so that she can have it all to herself.  

Abu ‘Umar said: This hadith is one of the best hadiths about the divine
decree, according to the scholars who have knowledge of the Sunnah. It
indicates that no one will obtain anything except that which has been
decreed for him. 

Allah, may He be glorified and exalted, says (interpretation of the
meaning): “Say: ‘Nothing shall ever happen to us
except what Allah has ordained for us’” [at-Tawbah
9:51].
So this matter is quite clear to the one whom Allah has guided, praise be to
Allah.

End quote from at-Tamheed (18/165) 

Al-Haafiz Ibn Hajar (may Allah have mercy on him) said: 

This indicates that it is emphatically prohibited for a woman to seek the
divorce of a co-wife, and emphasises that she should be content with
whatever Allah has decreed for her.

End quote from Fath al-Baari (9/127) 

Secondly: 

Undoubtedly her efforts to harm her husband, and her seeking help from the
kaafir police against him are also obviously haraam. It is not permissible
for a woman who believes in Allah and the Last Day to do such a thing, which
is the result of the Shaytaan toying with her. 

The same applies to her efforts to cause harm to her co-wife with regard to
division of the husband’s time, and to make the husband give her less than
is her right, or to make him incline towards the first wife, either for fear
of her efforts to seek divorce from him, relying on the kaafir laws that
will help her to do that, or to put pressure on the husband because of his
children, or other similar reasons, or abusing other weak points that she
sees in her husband and is exploiting in order to get something that is not
permissible for her, such as her husband being more inclined towards her or
favouring her with regard to division of his time, or trying to bring about
the divorce of her co-wife. 

The Messenger of Allah (blessings and peace of Allah be upon him) said:
“Whoever harms others, Allah will harm him, and whoever causes hardship to
others Allah will cause hardship to him.”

Narrated by Imam Ahmad (15755), Abu Dawood (3635), at-Tirmidhi (1940) and
Ibn Maajah (2342); classed as saheeh by al-Albaani. 

It
is not permissible for the husband to respond to her covetousness or her
wishes; rather he must strive to treat his wives equitably as much as he
can. However we do not think that he should let the matter go as far as
divorce; rather he should try to ward that off and put things straight, and
he should be keen to keep his wife and look after his children, and spare
them from living without a father, especially in that kaafir land. 

If
it is not possible for him to treat his wives equitably, because of the
circumstances mentioned, or for other reasons that may prevent him from
doing that, then we do not advise you to let your relationship with your
husband reach the level of all or nothing, by saying “Either me or her”, or
“Either give me my rights in full or divorce me,” or “Either this or that,”
and so on. 

Rather the wise person tries to achieve the best of two options and ward off
the greater of two evils, to achieve whatever he can of that which serves
the best interests and ward off whatever he can of harm, to the best of his
ability. 

Undoubtedly staying with a husband whom you love and he loves you, and who
finds comfort in you and you in him, even if that is once every three or
four days, is better for you than striving to seek divorce and live without
a husband, especially in a country like yours. 

The worst-case scenario is: imagine if your husband had four wives, not just
two, what would you do? Would you ask him to divorce all his other wives, so
that he would have more time for you, as his first wife is doing? Or what
would you do? 

Be
tolerant with your husband and give him a break, so that he can manage the
situation and solve the problem, for kindness is never introduced into a
thing but it makes it more fair-seeming, and it is never removed from a
thing but it makes it ill-seeming. When Allah wills good for the people of a
household, He introduces kindness among them, and if He wills ill for them,
He removes kindness from among them. Faith is based on two strong
foundations: patience and tolerance. Patience enables a man to bear what he
faces of the pains and troubles of life, and helps him to cope with
religious duties, and to resist the whims and desires of the self and
natural impulses. 

Through tolerance one will be able to show one’s good side and will be able
to resist whims and desires, and will strive to purify oneself and increase
one’s faith. 

May Allah make things easy for you, set things straight between you and your
husband, suffice you against the evil of that which is worrying you, and
reconcile you and your husband on the basis of the best of that which Allah
loves and is pleased with. 

And Allah knows best.