All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The jurists stated that it is dislikeable for a husband to call his wife with expressions or terms that make her forbidden for him, like saying: "You are like my mother for me." or "You are like my sister for me.'', meaning calling her "O, my mother" or "O, my sister". Their evidence is the narration in which a man called his wife: "O, my little sister!"; so the Prophet said in an objectionable manner: "Is she your sister? So he disliked that and forbade it. Thus, the jurists paralleled this to Thihaar (saying: you are like my mother for me).
A wife saying to her husband: "O, my little son" or other similar expressions, is similar to the above meaning, so it is more appropriate to avoid such expressions. However, these terms do not affect the validity of marriage.
Allaah knows best.