Siblings don't have standing to sue for custody, so you only have a few options: 1) Convince your mom to let her live with you and give you temporary guardianship. This is the simplest legally as it requires no intervention from the courts. 2) Have your dad sue for custody, win, and then take her in or let you take her in. If the plan is to have her live with you, this would likely not work out. Courts don't typically grant custody to people who are going to send the child away. 3) Report the mother to child protective services, hope they determine she's unfit, and then volunteer to foster her. If your dad declines, the court will likely give her to your custody, assuming you're capable.
A child cannot choose where to live. During a child custody hearing, the child's wishes play a role in the court determining the "best interest of the child", but they will not be the overriding determiner. See the site below for some of the factors that may be considered. Typically, the older and/ore more mature a child is, the more weight their wishes is given
International law is the body of treaties and conventions that specific countries have agreed to be bound by. Once signed and ratified by that countries government it becomes part of that countries law, jut as if it was a law passed by their own government.
The answerer above is wrong - there is NO international body that has the authority to impose a law on a country without its own approval.
For example, the Geneva Convention is an International convention governing the conduct of nations engaged in warfare. It applies ONLY to countries that have signed it, and ONLY when at war with another country that has signed it. So when the US attacked Iraq, the Geneva Convention applied to the actions of American troops, because both the US and Iraq were signatories. Had a US soldier done something that was NOT a crime under US law, or an Iraqi soldier done something that was NOT a crime under Iraqi law, but that DID violate the Geneva Convention, he could have been prosecuted for it, because the Geneva Convention was signed into US law by President Chester Arthur in 1882, and ratified by the US Senate, (Later amendments and modifications have been signed and ratified by other Presidents) and had likewise been signed and ratified by Iraq.
However, when Israel attacks Syria or Lebanon, the Geneva Convention does not apply to Israeli troops, because Israel has not signed or ratified the convention, and nor does it apply to Syrian or Lebanese troops - even though their countries are signatories - because they are fighting against Israelis: troops from a non-signatory nation.
(Note that even though Israel has not signed the Convention, there is an Israeli domestic law that requires their troops to follow most of it's provisions anyway. Israels main reason for not signing the convention is its clauses about refugees. The convention requires signatory States to take all reasonable steps to allow refugees to return to their pre-war homes as soon as possible, and prohibits a winning country to seize the homes and lands of civilians from the losing country - both of which clauses are, of course, contrary to Israeli policy.)