The Bride has been adding labels to her blogs and has encountered a problem. There are two Sisters of The Bride and they are both important to her. She does not want to name them Sister #1 and Sister #2. If you don't immediately understand why, then you do not have siblings.
Younger and Youngest don't appeal either, because The Bride doesn't need to be reminded who is the oldest.
So, henceforth, Sister Pink refers to the Midwestern Sister (because she looks pretty in it) and Sister May refers to the Western Sister (because she was born then and the colors she looks pretty in don't work). Sister Rust? Sister Dark Bluey-Green? I think not.
The Bride also has two brothers, but she will cross that bridge when she comes to it.