saw off the cantilever back to near the beam or posts. then develope a new ledger board using hangers. Where the ledger board is built, install a new post and beam system there. Then bulld a new post and beam section 6 feet out from that and then again every 5 or 6 feet.
Continue your joists boards straight off from the old ones so that your nail line is again straight from the original.
With this method, you will have two seperate decks tied together by a good post/beam/ledger system.
This should do it.
I would not recommend sistering. Remember, this is my opinion and there are many ways to build a deck.
Sistering makes for an off set nail line and my customers expect and demand a straight string poping nail line.