Sister and brother

We went to our friend's cafe because she thought she saw their daughter and she wanted to play with her. The girl she saw wasn't her and she started playing with her brother. I was having coffee and talking with my friend and when I saw their backs side by side looking outside, somehow I felt their bond as a sister and  a brother. I can't see their faces but I could tell that they are a sister and brother, not because I know they are.