Splitting up a piece of land or a second home amongst your siblings and in-laws can get messy if nobody can agree to be an adult about it. Your siblings or in-laws might reveal the raging entitlement they kept hidden for so long beneath their possibly faux niceties.
The most effective course of action is to reach out to them and try to understand why they booked your grandparents' winter cabin every single weekend in the winter. Possibly, ask them why they thought it was okay to call dibs on a shared home and not blink twice about it. When they retort back with some excuse that further proves they think their needs should be placed above yours, retreat. Call the grandparents. Let them know what's going on and try to remedy the situation that way.
Even when your grandparents put their foot down and divvy up the weekends evenly, the entitled in-law will try to force you and everyone else's hand to give up the weekends reserved for all of you. Grandma and Gramps refuse to believe that someone like this has married into the family, so they revoke her privileges completely. Sweet, sweet karma. Scroll to read the whole story.
