My mother saw my sister leaning on my shoulder and whispering to me. Why is my mother so boring when she says sourly how old you are?

My mother saw my sister leaning on my shoulder and whispering to me. Why does my mother say sourly how old and boring you are? This time, my mother thinks that you are both too old to be close. My mother told you that this is not good. Don't do this in the future. I think that's what she meant.