Yes,Mannu is Ratan and Ganga's son.In Jagya-Ganga-DS-Anandi track a child marriage happened and in other side of story Nandini's child marriage happened.Shivam and Jagya-Ganga's son Abhi is not yet married,probably two are enough for now.And BH family are totally against child marriage so no question of Shivamor Abhi having child marriage.
Anandi didn't sacrifice her daughter,yes,but when u see Pooja willingly,knowing all the issues regarding child marriage decides to get married to Mannu then it comes to one's mind that Pooja should have been married and Anandi shouldn't have stop that marraige.Anandi lots her daughter to save a girl from child marriage and that same girl gets married when she is a kid.Then Pooja had the audacity of agreeing with Mannu in front of all the elders that they should live together as husband wife so yes,it was kind of a sacrifice.The life Pooja is leading is of Nandini's.