J.M.
Seeing as your sister is in California, you may be able to check into a diaper service. If she's going to cloth diaper, there are some really great services that will come on a schedule and pick up the dirties and bring you clean ones. The cost is determined by how often they come and how many diapers are needed - and believe me, she will need a lot of diapers! You could also send her a gift certificate for an online baby store such as www.diapers.com, which delivers right to your door so you don't have to leave the house to get diapers or wipes, and they're really reasonably priced. The best thing that I received was my Graco DuoGlide Double stroller - it's a tandem and a bit long, but after a couple of times using it, I had no problem with it. It can hold two snap-in car seats, and when the babies get older, both the front and back seats recline for babies to nap while going on walks or to the store. Things like that are a bit expensive ($175-$450 depending on brand), but can be ordered on-line and delivered right to their home.
Please let your sister know that having twins is hard, but at least she's doing it the first time around! I wish that I had twins right off the bat, instead of having one, then twins, and now another on the way! My sister also has twins (with three older children), and says the same thing! Your sister is lucky - she will adapt to it much easier than those of us who had a singleton first....mostly because she won't know any different! Congratulations to your sister, and good luck to you on TTC!