A way I look at Karin and Sakura as opposed to Ino and Sakura is Ino is more like a big sister to Sakura, someone she squabbles with but invariable they always back each other up and love each other dearly.
Karin is like a True friend, someone she speaks to as an equal no silly rivalry simply two friends with lots in common.
Both Have Naruto and Sasuke in common both are med nins both are feisty and fiery and both stand firm by their loved one and if needed stand in front of them