She's a girl.
Well, no. She is on the cusp of womanhood, a hybrid of girl and woman. You can tell that she will grow into a beautiful woman once she finishes her journey of adolescence, the way a sprout develops into a lovely flower. She is much younger than you and yet, at the same time, your age. She is your best friend, your only friend (for all you have is a mate and a family) and she is to become your sister one day. You know that it is not wrong now to dream of girls, to wish to kiss - hold - hug - love one as you would a boy, but you are married to a wonderful man, your mate - you have been long before she was born.
You should be sensible for once, darling. You have a husband, and here you are lusting after your brother's mate. You're straight, really, it's just a passing fancy. But her long brown hair and wide eyes the exact same shade -
No, best not to go further down that path.