Ruri is moving, her older sister is there so she can't be going to college and everyone in her family is going.
Text evidence 1: I didn't want my older sister to see me crying.
"It's almost time, Ruri," my mother said gently. Her face was filled with a kind of sadness I had never seen before. This shows they are all going and if she was so sad she could just not go.
Evidence 2: Her mom was also sad, if she were getting married I don't think her mom would say "It's almost time," I think she would say "Are you almost ready for your big day?"
Evidence 3: If her parents were being divorced ALL her stuff wouldn't be packed.
In conclusion the only possible and efficient answer would be 'Ruri's family is moving to another place.'