I know a family of four girls (none of them twins), and you absolutely cannot tell two of them apart. All four of them look almost exactly alike.
There are baby pictures of my older son wearing a hat that look *identical* to how my youngest looks now. My youngest has much lighter hair, but everything else is almost spot on. I don't have any baby pictures of DS on the computer (he was a baby before digital cameras were popular!), but see if you can see the resemblance between them now:big brotherlittle brother
However, I don't think my DD looks anything like them AT ALL.