A second opinion visit is standard (or was when we adopted quite a few years ago now).
The manager wants some things to be checked before going to panel. She presumably thinks that some areas of your PAR are weak. This might be because it is poorly written, or because your SW hasn't explored things properly, or because they are actually weak.
Either way the manager rightly wants things to be as solid as they can be before submitting things to panel, as if panel spot a weakness then you will be deferred. So better to make a strong case upfront.
So family set up - I'm guessing maybe it is unusual fro some reason? In which case it needs to be clear to panel (and placing SWs) that adoption will work for the setup. No point getting through panel if no one will place with you.
Evidence for sibling group. Again you need to show you have considered implications, that your support network is strong enough etc. The manager may have some concerns that the PAR is not showing you to be robust enough.
Try not to panic. Panel may be delayed by a month, but that is unlikely to affect matching, since if you are that close to panel they may well be considering you against children anyway. (We were approached about a match a month before we went to panel).
It's very stressful but will probably all be OK.