My stepdaughters are 2.5 years apart and they didn't like getting the same thing. So I'd get different types of gifts for them. The mani/pedi is nice but it doesn't last so maybe skip that. The older one might like a gift card to a bookstore - she can get a book, CDs, gifty things, a big range. You could buy a book and talk to the store about a gift receipt - some stores won't do it because people read the book and then return it, but see what they suggest. If you know her taste, you can buy a book and gift her a card for the balance. For the younger, what about earrings or a necklace or scarf? A lot of kids like those because they can make their "ordinary" clothes (even a plain sweater) look trendier. You could also get either one a sweater - don't get too trendy for the older one.
I wouldn't get any more personal than a sweater - and if that's too much, then go for the other ideas.
Another option is movie tickets - we get discount tickets through AAA (usually 4 or 5 in a set, and for any of a number of theater chains) and then we split them up to give as gifts in sets of 2 or so (depending on the size of the family we are giving them to). If there's a spare one left over, we keep it for our own use!
As always, you can ask their father if they're nagging for anything, and then come through and deliver that!
Depending on your relationship, you could take them shopping too - and maybe grab lunch together. But the stores are starting to get crazy insane and one may be bored while the other is shopping (where you need to keep the price under control too), so think that one through.