Ethanryan, your neices sound so adorable ! the scrapbook is a wonderful idea from the other girls. I do lots of scrap books and have a decade of them right from the plastic pregnancy test with the little pink plus sign in it from the day I found I was pregnant with my little girl (now almost 11). You can put all sorts in scrap books, you could even back them with wrapping paper that you can get from Woolworths and WH Smith with their names on (or even print your own from your computer). Start with putting foundation on their little hands and making skin coloured hand prints of their hands - foot prints too, with the names and ages next to them. You could put post cards in there of pictures of where they live and even a letter in an envelope each for them telling them how much you love them. Keep everything, cinema tickets, bus tickets, everything, their memories will be captured every time they see the book.
The other thing I was going to suggest was a Digital Photo frame with you having loaded up nice pics of you with them for them to put in their bedroom. Keep the box etc at your house with the receipt etc in the event that your sister may decide to sell it for a few pounds.
Do you live far from your sister ? When will you see your neices next?
Oh, I have just had a thought, Asda have some �5.00 mobile phones in stock, perhaps you could get the 7 year old one of those for you to ring / text your neices to it and you could perhaps top it up for them with your debit card by �5.00 a month so they could send you texts too ?
Katie. x