The possibilities on 'adopted child' according to me,
1) It could be a Kapadia sibling's internal joke, their thing like Muqqabala that they had been doing since ages.
Sab ko shock karne ke baad hasne lag jaaenge then family reunion with Kapadia family pic.
2) If it's true, Anuj might felt the need to give emotional security as well as financial upper hand to Mukku so that he ends any possibility of her leaving him again.
3) Instead of questioning him, Anu would be more empathetic to him and more emotionally drawn to his pain. Kind of dard se rishta gehra karna and all. Uski toh adat hai sabke dard mitane ki. Basically to give more depth to MaAn connection (as if it is needed)
Chalo Meri bakwaas hogai! Baki tum Sab apna apna dekh lo! 😜