Complex sentence: Princess Vera lived with her mother and her brother George for the next two years in Sweden, first in Stockholm and then in Saltsjöbaden, but Sweden was too expensive to live in, so they moved to Belgium by invitation of Albert I of Belgium.
Split into simpler sentences: Princess Vera lived with her mother and her brother George for the next two years in Sweden, first in Stockholm and then in Saltsjöbaden. As Sweden proved too expensive to live in, they moved to Belgium by invitation of Albert I of Belgium.