Conrad, if Susan adds her name to the deed, the mortgage is due.
Susan, Your best bet is to do a FHA which will allow your sister to sell to you the house for the full value and give you a gift of equity for the difference between the sales price and the mortgage payoff. You can also get a gift of equity for the closing costs (if there is enough equity). At the closing your lawyer will arrange for the transfer in the deed. The title company will file it for you.
I would suggest you use a different real estate lawyer. It is not uncommon for someone to live in NY and commute to Ct even if it is Canarsie.