I think on the one hand, it is love, mutual understanding and mutual care. I think they will get married in the near future. I wish my brother and sister both married, loving and happy forever.
On the other hand, it means brother and sister, which can be really related or called casually. Sometimes calling a person "brother" is similar to saying in Mandarin: "Please! Big brother (expressing helplessness). " The feeling of "big brother" inside.