One thing i really loved about Mikasas story in lost girls is the revelation that her mother was pregnant with another child, ad that Mikasa was going to have a brother. If her parents never died and if she was still living her quiet and peaceful life with them, she would have had a sibling to take care off. But because of her parents deaths, she lost all of her family in a single moment. She didn’t have anything else to protect or live for, until Eren came into her life. In alot of ways, Eren represents the kind of sibling love and devotion that Mikasa would have had with her unborn baby brother if her mother hadn’t died. They play and get along with each other, and bicker like siblings do, and i think Mikasas mother kind of acknowledge that and that’s why she said they looked so much like siblings. In many ways they kind of are because they act like siblings.