Detailed analysis of the first meaning: there is a kind of kinship between sisters with blood as the link. After the sublimation and accumulation of time, this kind of affection has finally become a kind of affection, no less than the affection of parents for their children: an elder should take care of his sister, love her and care for her like an elder. Sister is proud of her sister, and sister is proud of her sister.
Although their parents are not at home, they are sisters.
Specifically analyze the second meaning: "Sisters are affectionate", which means being as affectionate and righteous as sisters. That is what we usually call an emotional confidant.
Example: Although we come from all corners of the country, we are sisters.