OP, you are a joint-owner of this cottage property. While you may be the majority owner, you still need to share decisions with the other owner -- you cousin. First order of business is for you and this cousin to sit down and discuss exactly what are the ground rules: How to determine who gets the cabin which week/weekend, when are "guests" allowed use of the property (must one of you be present, can you "lend" it to a friend to use during "your" week, etc.), what are any plans for maintenance and improvements both near-term and longer-range plans?
Once that has been set in writing and agreed to by both owners -- then comes the discussion of your brother and his boat and his "plans." Did the cousin say he could leave the boat there? How and when did the boat come to be docked there? It's January now, so presumably this all occurred several months ago (back in the summer). Ideally that would have been the time to address it, but better now before spring boating season. However, IF your joint-owner/cousin allowed your brother to dock there, you can't just override that decision. Hence, paragraph 1 above to hammer out ground rules between the owners before dealing with your brother. "History" doesn't grant ownership or access rights, it provides memories - hopefully good memories for your brother.
Good luck!