We have had many families of 5 taking our Lotus train service.
The most preferred option is to book 2 private cabins, one is the VIP 2 berth cabin and the other is the VIP 4 berth cabin. These 2 cabins will surely be next to each other.
the less preferred option is to book 5 tickets in the 4 berth cabin. They will get 1 private cabin and the 5th person is usually in the next door cabin.