If you are both staying in the same room at a Disney owned resort, your name should already be on the room reservation and you will be able to take advantage of DME at no cost to you.
If your name is not on the reservation and your sister is checking in to a Disney owned resort a day before you arrive, then your sister can add your name to the reservation when she checks in. She would then need to contact DME and let them know you have been added to the room reservation and give them your flight information for the next day.
This is what I was told to do by several CM's at both DME & the resort I am staying at. I am arriving on one day and my sister is arriving a few days later. I cannot add her to the reservation before I check in, as I am booked with a package and she is not (already has a park hopper pass). Was told to wait until I check in to the resort, then add her as a guest in the room, as you are allowed up to 2 adults in a room at no extra charge. Once I add her to the reservation, I just need to call DME, they will show her as a Disney resort guest and give them her flight info. She will not have tags OR vouchers ahead of time, but just needs to go straight to DME (she's sharing a suitcase with me) and they will print her out a voucher at that time, then grab the next DME bus to our resort at no extra charge to her.
I will be traveling alone and can't use DME as I did a quick plan change and am arriving the day before my reservations (sister has room for the night). I thought I could use Mears shuttle but have found they don't 'do disney' (SSR) due to ME... from what I've read here, it seems cabs could be a little less expensive for one person but how safe are they? And where does one find 'legitimate' taxis and avoid 'gypsy' cabs? Is it worth the extra money for a towncar? Thank you all and 
sorry if something similar is already posted - somedays I can't seem to be able to put the right combinations of words into the search strings.
