^ And now Christopher will surely point out that no one thought Luke and Leia were siblings at the time of their casting in 3... 2... 1...
Well, further more, no one KNEW Luke and Leia were siblings when Episode IV or even when Episode V was made.
Lucas didn't pull that out of his ass until at least the third draft of Episode VI written the night before filming began.
I think "requiring" actors playing siblings looking a like is too much, it's pretty much never happened at all in all of media. We could probably sit here and point out numerous and countless places where it's not happened.
Obviously they should at least be the same skin color and stuff like that but that's probably as far as you should go. I had no problem accepting Susan and Johnny Storm as being siblings on looks alone. In the way the interacted between one another? A tad. In the way Susan acted and was supposed to be the older sibling? Sure.
Looks? Never entered my mind, siblings looking different is one of those Hollywood tropes we deal with. Like actors in their 30s playing high-schoolers, it's just part of the way things work in Hollywood.
You cast people based on how well they can play the part and also maybe in how much of an audience they can draw in. If they felt Alba was the best woman for the job they weren't going to turn her down because she kinda doesn't look like Chris Evans.