Hi there!
I want to start buying some things now to help out (and some is just too cute to resist!). I am thinking they will need a lot of diapers and wipes so I thought I could pick them up here and there when they were on sale. What do you ladies think of this idea? What sizes would you recommend? Anything else you needed a lot of?
ETA: Thanks in advance!

Re: Brother/SIL having twins
It's hard to plan ahead with diapers....they may find that some brands don't work well, or may cause skin irritation. Wipes are a little easier to predict, but probably the best thing to do would be to add a $20 gift card every so often, then you'll have them to give her as she needs them, or to use yourself on diapers once they figure out what works.
You could also start buying sleepers/ onesies in various sizes, but again, that's hard not knowing what the parents/babies will prefer.
Our crazy, wonderful life
cook for her! bring her meals that she can freeze and just pop in the oven. when you go to her house just start helping...don't wait to be asked or ask what you can do just wash bottles that are dirty, tidy up toys etc...
Don't push her to leave her babies but offer to let her nap. I also just loved it when someone stopped by with a coffee and just held a baby and talked to me!
diapers, wipes are good, white oneies in various sizes. baby advil, tempra, gripe water etc... all stuff that we used//use often and is expensive.
We've used Huggies and Pampers all the time, for the past 5 years! (have a 5yr old and twin 2.5 year olds) We have tried the Target brand Up & Up and HATE them! We tried the WalMart ones with my oldest and hated them as well so thats why we've just stuck with Huggies and Pampers. We used a lot, I mean LOT of size 3 diapers. I think my twins were in that size for a good year or so. I would maybe buy that size rather then too many Newborn or size 1's. Espically if they are having a shower later on a lot of people might buy those two sizes.
Sleepers are always a good thing. Button ones were my favorite over the zipper ones. Bibs or burp clothes were always useful as well! Blankets blankets and blankets!! (espically if you are in an area thats cold) It seems like we couldnt have enough with them spitting up on them. Babies R Us usually does a good sale with Buy One Get One 50% off
Have fun being an Aunt to twins!!!