What your TA "called" a family cabin but was not really according to MSC's terminology?
If it indeed were just two cabins you booked your TA might have better explained to you the real facts, what you booked and the T&C of MSC.
There have been a number of similar posts before and the base line is (Sorry for not having an answer more likeable to you): Whatever your TA might have told you, MSC always has the right to change your cabin numbers. Thus if it is one (1) family cabin it might be moved but should stay together, if there are two (2) cabins booked, they might be moved independently. It has happened in the past, it now has happened to you, and most probably it will happen in the future. Rarely, but sometimes.
Nevertheless, good luck in your attempt to get it changed.