The cousin who books our lake week now acts like he owns every bedroom

For nine summers, twelve adults in my extended family have rented the same lake house for one week. My cousin Mark handles the booking because the owner knows him and the reservation opens while most of us are at work. We all reimburse him through Venmo within a few days.

Somewhere along the way, booking the house became owning the house.

Mark and his wife take the bedroom with the lake view. His brother gets the room with the king bed. Mark assigns the rest in a group text after everyone has paid. Last year my wife and I got twin beds beside the laundry room even though we arrived first and paid the same share. When I suggested rotating rooms, Mark said the organizer should receive some benefit for fronting the deposit and doing the paperwork.

That would be easier to accept if the benefit had been discussed. It was not. He also does not absorb the financial risk for long; all of us pay before his credit card bill is due. The paperwork is one email chain and a signed form. I am grateful he handles it. I do not think gratitude automatically includes nine years of first choice.

This year the problem expanded. My aunt recently had knee surgery and asked for the downstairs room. Mark assigned it to his brother, who likes being near the kitchen, and told my aunt there is a sturdy railing upstairs. She is now considering skipping the trip.

Several cousins want me to challenge him because I am the one who raised rotation last year. None of them wants to take over the reservation. Mark knows this. He says if people dislike his system, somebody else can book a different house.

I looked at alternatives. Comparable places are limited for our dates, and changing houses would probably split the group. I also understand that invisible planning can look easy to people who never volunteer. But a task can deserve recognition without becoming a private claim on shared space.

What is a fair system here? Should the organizer receive a smaller discount, first choice every other year, or simply thanks? And how do we change the room assignment before my aunt drops out without making it sound like we only noticed the system when it affected someone visibly?