My guess was going to be 2 or 5, and I was leaning towards 5 because that looks like family in a picture to me. Sorry, but you do look a lot alike.
ETA: I should mention I did one of the internet face recognition things, and I got really high marks, so you should listen to me. Seriously. 
Well!
You and your sister don’t look dramatically different, but not particularly similar either. I really thought there was a clear family resemblance with #1!
Huh.
My guess was going to be #2, though I thought that most of the ladies pictured had a resemblance to you. #5 was second to my last pick (#6 was the last).
I guess that I can pick out a few similarities now that I know who it is, but it’s certainly not obvious. Sorry, Mom. 
Facially, I’d say number 5…but number 2’s smile is more similar.
I’d hedge my bets between the two, but veer towards 5.
ETA, after viewing the rest of the thread and spoiler: Woot! Go, me! (And, also, featherlou!)
We (people in general) probably need more photos from more angles to make a good judgement call. The smile from #2 is really similar to the way you’re smiling, but maybe you look much different without smiling. The reason I picked the three I did is because of the noses, and it’s a bit difficult to see your eye and cheekbones because of the veil.
If you want a more definite answer for your mom you need to set up a slightly better test.
I guessed #5 immediately.
You’d think her mom would know, she was there for the birth.
Number five. I’m saying that less because the two of you look similar (you really don’t) and more because your demeanor is so different in that picture compared to the rest of them and that girl’s demeanor is so different from the girls in the other photos.
I wonder if you two get along very well.
Yay, I’m awesome!
I agree with most of the rest of the thread as far as looks go; number two looks more like your sister than any of the other women.
I say #2.
Joe
Hey, I thought bridesmaids were supposed to wear really ugly green dresses!
Ok. I haven’t read any of the other responses, but I’d say it’s between #2 and #5. There’s a definite resemblance between you and #2, and if she’s your sister you wouldn’t have told your mother you don’t look alike. I’m also struck by how the pic with #5 is the only serious one . . . like it’s a family wedding photo, something you know people will be looking at for a long time in the future.
So . . . if it’s #2 your mother’s right. If it’s #5 you’re right. I say #5.
The OP seems to be questioning the mom’s honesty.
!!!
I sure didn’t read it that way!
Anyway, it seems Apidistra and I are the only people who think you look like #1. Doesn’t anyone else see a resemblance there? Ignore the haircut!
More angles being requested (and these pics are over the course of a couple years. I may look more or less like one or the other as my weight fluctuates):
http://flickr.com/photos/31466949@N05/sets/72157608109419031/detail/
http://www.flickr.com/photos/31466949@N05/sets/72157608097844150/detail
The three of us together:
http://www.flickr.com/photos/31466949@N05/2948153121/
I don’t think my sister’s not my sister. For one, once you see the whole family together it’s obvious: http://www.flickr.com/photos/31466949@N05/2949005456/
As for the unsmiling picture-- We get along great and I love her to bits. She just tends to look uncomfortable when she knows she’s being looked at by stranger. It was a hard day for her. She is very, very uncomfortable with crowds, strangers and disruption of her routine (She’s a Aspie). She did so incredibly good at the wedding, but I know it was difficult for her to do all the smiling and small talk. (She’s happy to spend her life in a lab with her petri dishes all day and talk to no one. Would drive me bonkers).
Mom has some 'splainin to do. You and sis don’t look anything alike.
I hate to be contrary, but your sister is clearly the most similar to you from all those photos. Even with the handicap of her glasses and your veil.
#5 has almost exactly the same cheekbone structure, once you know the answer.
Is there going to be a swimsuit portion in this competition. I think that would really help us decide.
My guess was between 2 and 4 but I was going to go with 2.
Number 5 - number 2 is quite similar, but you and number 5 have the same set to your mouths, and the same milky skin.
Given the pictures on this page, it does become more likely you’re sisters. Especially the first photo and the second number 3 photo. But I still don’t agree with your Mom that it’s “obvious”.
I also wondered, just on behavioral cues, if number 5 might be the sis, but didn’t mention it because you wanted to know who you *looked *most like, not who could play Sherlock Holmes. But I’m glad we suspicious folks were wrong and the reason for the change in demeanor was a loving one, not an angsty one!