Product backlog items not defined in time and not sized up

Hello, In the organisation I work for, the product owner does not size the PBIs and defines only important backlog items. This creates problems during sprint planning sessions as some of the features are dependent upon other, and some user stories have to be defined during the session to support the core development activity. Since user stories are not sized properly, the team invests a lot of time during sprint planning to size up the stories and estimate them. What is the recommended way of dealing with this situation?

Posted By - Christian