You should call her by her first name. You may refer to her in third person as your sister-in-law.
probably by her name
Sister In law
A hoe.
aunt
We call "Mami" to moms brother's wife.
In the English language there is no difference between the wife of an elder brother and the wife of a younger brother or of a twin brother. The wife of any brother is your sister-in-law. If you are actually talking to her, however you should just call her by her name. Nicknames are fine too.
sister in law
Shwe would be your sister-in-law.
Brother-in-law
Pick up the phone
errrr, no maybe nephew
Your sister-in-law.