We could tell from the ultrasound pictures that our two girls (34 months apart) would look like each other. I have actually had several people ask me if they are twins (they do like to dress alike, which I'm sure helps with that appearance), even though they're not nearly the same size.
The youngest does seem to be growing a little faster than her petite sister though (at 3 & 6, there's only 4 inches difference in height). The eldest is leaner in her facial features, while the youngest has a more rounded face (if that makes sense... it might just be left over chubby cheeks).
I went back and looked... my sister and I are 8 years apart, so as kids no one thought we were twins. But as adults, we bear an amazingly striking resembelence to each other and our aunt. My sister will often walk into my work place and say "Have you seen me?" to determine if I am at work or not (but then, she's a smartalec). I went back and looked at pictures when we were kids, and my girls look like me sister, which means they look like me (for some reason I don't have ready access to pics of me as a kid. weird)
For my girls, when I look at a picture of them, I have to look at other visual clues to figure out who it is... we didn't get that outfit until #2 was born, so clearly it's her... or so-and-so is in the background which means this picture was taken at such-a-date, so it must be #1.
Weirdly, my girls look like they could be in the family of our best friends, who have two boys and a girl (ranging from 10 down to 2). Hubby and I are brunettes (now, but were blonde as kids) and both our girls are blonde! Our friends kids are all blonge (very norwegian looking), and I just looked at a picture of ME when I was about 2 and thought wow, that looks like Baby J (our friends 2yo daughter).