The Memory of Earth (1992) by Orson Scott Card, first book of the "Homecoming Saga"?
From Goodreads:
High above the planet Harmony. the Oversoul watches Its task. programmed so many millennia ago. is to guard the human settlement on this planet-. -to protect this fragile remnant of Earth from all threats. To protect them. most of all. from themselves. The Oversoul has done its job well. There is no war on Harmony. There are no weapons of mass destruction. There is no technology that could lead to weapons of war. By control of the data banks. and subtle interference in the very thoughts of the people. the artificial intelligence has fulfilled its mission. But now there is a problem. In orbit. the Oversoul realizes that it has lost access to some of its memory banks. and some of its power systems are failing. And on the planet. men are beginning to think about power. wealth. and conquest.
Specifically, from this review:
Look, this is what Card thinks is a matriarchy: exactly the same as the US, circa mid 1980s, but only women are allowed to own residential property inside the city proper, there's a special religious lake that men aren't allowed to visit, and people sign marriage contracts with each other that only last one year.
Wikipedia's character list has the brother being an actor, and his name begins with M:
Mebbekew
Nafai's second oldest brother. An unabashed hedonist and would-be actor, his most recent seduction at the start of the expedition is Rasa's niece Dol, a former child actor.
The cover has someone on a floating chair:
Found with the Google query scifi book marriage lasts for a year site:goodreads.com/book.