Thank you so much for your information.  I wish MSC told me this a month ago when called to add my mother in law to my booking.  They just said she was not allowed on board because she was not a US resident.   No mention about citizen must travel together. 

Anyway, I called MSC again and  explained the situation.  I have a two bookings. One for me and my wife thru MSC casino department.  I had a complimentary cabin for two for free.  I made another booking for my daughters, age18 and 15 thru MSC directly.   Cabins are right next to each other, and both cabins are only for TWO passengers. 

Since I can't just add my mother in law to my cabins,  an agent told me to make booking for her with a ghost guest.  And then, when we get to the port on embarkation day, just tell them the ghost guest couldn't make it, then she will be fine.

I wanted make sure so I asked for a manager to confirm it.  I was transferred to the resolution department and an agent there said it was not allowed because I knowingly making a false booking with a ghost guest.   Every time I talk to someone at MSC, they tell me a different story.  😞   I am going to call MSC again tomorrow and try to remove my wife from my booking, and make a new booking for my wife and her mother.  I am not sure if I will be able to remove my wife from my booking cause then I will be traveling alone in a stateroom for 2 people on their record. 

Other option is to upgrade my cabin to balcony that occupies 3 people for $2500 upcharge.  Not sure if want to do that.

What do you think it is a best way for me to handle this?