[Nyororin]

It will probably be easiest if you stick to Japanese style rooms - hotels with 5 beds in one room would be almost impossible... But 5 futons in a large Japanese style room would be no trouble at all. Ryokans are almost entirely Japanese style, and in my experience hostels also tend to be so - although a hostel may not have a room large enough to accommodate all of you.

If you stick with ryokans and minshuku (a sort of lower level of inn) and tell them that you have 5 people in your family, you shouldn`t have any trouble. At the in-laws` inn, they have groups of 8+ stay quite frequently - it`s very easy to add another futon to a room.