I would appreciate your advice on the fair thing to do

I agree that it should be split by 6 and lots drawn for the rooms. Maybe there could be a slight reduction for someone who ends up in a box room, particularly if the double rooms have their own bathrooms and better views.

Everyone in the house will use the shared areas, and you'll be in them whilst awake far more than in the bedrooms, so it is very unfair to expect single people to subsidise couples. If any 'coupled up' people don't want to share for any reason, maybe get a bigger house with a room each? Otherwise I'd assume that sharing is a preference and not something that should be discounted or paid for by the singles.

All bedrooms are en suite and, as far as I know, very similar. Thanks for all your comments; I am inclined to agree that it should be split 6 ways too.

Six people each paying the same for the holiday would be my choice. Are the bedrooms identical though? When we went to a holiday house with a family group we payed the same but drew lots for the best bedroom.

Split by 4. A room is occupied times 4 regardless what goes on in it. Unless of course the double ones have ensuite bathrooms?
Food split by 6.

6 ways

Split it by 6 would be the way to go I’d have thought.

Split it six ways.

I would split by six.

I would say split it into 6 as there are 6 of you. Others may disagree. See what the other sisters think.