Need to remodel my itinerary, help please!

Why would you have to reschedule your flights? Are you still going on the Italy tour? They are excellent for solo travelers!

Did you book directly with the airline or did you go thru a 3rd party? Which airline?

Is your travel partner listed as a beneficiary on your trip insurance?

As far as changing flights?
Why?
Depends on what sort of ticket you booked-we usually fly United-no change fees
Not sure if your trip insurance would cover any cost difference unless partner is beneficiary-doubtful

First time RSE tour to Italy.

Based on OP other post it is the VFR starting Sept 8.

If you are still going why the need to 'reschedule all flights'? ; or need to use your trip insurance?

Did your sister buy trip insurance? If she is the one not going, she is the one with a claim to make.

Would someone please point me in the direction as to where to start? And what documentation I'll need to provide to insurance co. and airlines?

Contact the insurance company and explain your situation and have them tell you what they require. Same with the airline.

I'm just blindly guessing, but are you still planning to go on the RSE tour to Italy but were going to go a few days before the tour and stay a few days after and now want to go only on the tour? If so, using your trip insurance might not be necessary if you can cancel your lodgings without fees and do the same with rescheduling your flights. I would contact the airlines before doing anything with trip insurance. We don't know the particulars. Can you clarify?
I really feel for you! It's so hard to have plans and then have them evaporate.