Monghi is definitely not khemi and santu's sister.
Nanaji: Taru and DM's dad.
Mother: Yes bapu, she is about to say something but she stops.
Nanaji: What are you hiding form me?
Mother looks at the child then says: This child is his grandson.
Nanaji is shocked
Mother: Tarulata and Kanji's child
Nanaji shocked even more: Tarulata's child, what are you doing, if they find out you won't be spared.
Back to the present
Nanaji: They left immediately to return the child but the bus crashed and no one survived.
Santo is shocked as she was over hearing the conversation,
Kanji devastated,
Nanaji: I hid this from you so you won't be upset, but Kanji for looking after my granddaughter, I didn't want you going from corner to corner looking for your child.
The above is the 11th august update portion. Even Maulik seems too old to be his son as he does seem to look at least 18 years old.
As no one survived the crash, that means the santus parents and the child died. At the time there was only santu and khemi. so munghi is definitely not their blood sister. Even if khemi is married, it cant be DMs lost son who might have survived coz he would only be about 14 years old now.
Perhaps the nana is still not revealing the facts. maybe they did take the child back and taru's father may have taken and put him in an orphanage or boarding school but no one knows about it.could have made them swear never to reveal to anyone esp kanji, and that child may still come up in the future. In that instance, then santu's parents could have come back and had mungi and have died at a later date.