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. Add one cabin to the cart then back to search again to add another cabin to the cart, then check out.
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.