Your brother's wife is your sister-in-law. It is appropriate to call her by her first name, however, if you want to use a diminutive of her name (ie calling Elizabeth Beckie), please check with her first. Some people are not comfortable, nor do they like "nicknames".
In the English language, no distinction is made between the wife of a younger brother and the wife of an older brother. The wife of your brother is simply your sister-in-law.
A hoe.
aunt
We call "Mami" to moms brother's wife.
sister in law
probably by her name
Shwe would be your sister-in-law.
Pick up the phone
Brother-in-law
Sister In law
errrr, no maybe nephew
Your sister-in-law.
You would refer to your younger brother's wife as "bhabhi" in Hindi.