You're not doing Scrum then. One of the few meetings that Scrum dictates is "backlog refinement" where the dev team works with the PO to get the stories into a workable state.
If you can't touch the stories, you can ask whoever does to break it down. If they don't then you shouldn't ever pick it up into your sprint. If they insist and it doesn't work out then you should call out the incident in your retrospective and hopefully get everyone to agree to do things differently in the future.
Sadly, it’s more like the dev team points out that we really cannot work with these stories then management telling us to deal with it while the BA’s suck their teeth.
