Deviation Actions
Sarada and Karin have nothing to do with each other and this has officially been confirmed in the actual manga. ._.
Hope you've already read Naruto Gaiden Chapter 10, in which Sasuke, Sakura and Karin themselves confirmed that Sakura is, indeed, Sarada's real mother:
-> Firstly, Sakura herself said right away that she is Sarada's real mother.
-> Secondly, Sasuke himself told Sarada that she is the living proof of him and Sakura being connected, thus basically stating that she is the living proof of him and Sakura loving each other. After their marriage, he and Sakura decided to transform their love into a precious child. If he and Sakura hadn't loved each other, Sarada wouldn't have existed. She wouldn't have been born. But she EXISTS and her simple existence shows the deep connection between her parents. c;
^ This is what Sasuke truly meant while saying: "Because you EXIST, Sarada.". (This is what he truly said in both the raws and the Media Viz translation, which, as I've already said, is the ONLY official translation. Thus, in the online translations, his words were WRONGLY translated as "Because we have you, Sarada.")
^ Sarada immediately understood Sasuke's words perfectly and, thus, started crying happily right after hearing them, as this was the ultimate confirmation she needed in order to know for sure that Sakura really was her real mother! c;
-> Furthermore, Karin herself also confirmed that Sakura is, indeed, Sarada's real mother, that the umbilical cord was between Sakura and Sarada (this being the reason why the DNA test was 100% positive, as an umbilical cord contains only the DNA of the child), that she was only the one who had helped Sakura give birth to Sarada (thus, being the midwife) and that she had only wanted Sasuke to be happy (thus, basically admitting that Sakura was he only one who could bring happiness to Sasuke). 
^ Therefore, Sakura being Sarada's real mother is a FACT that was confirmed in the actual manga by Sasuke, Sakura and Karin themselves and, thus, Karin literally has nothing to do with Sarada. c: So, I really don't see why you're trying so hard to "add" her to Sakura and Sarada's family, when she literally has nothing to do with them apart from being the one who helped Sakura give birth to Sarada? -.-"