Does my sister have a right to be upset?

yes she has the right to be upset. absolutely.

You sort of changed your story.  First you say you assumed she wasn't interested.  Fine, but clearly this was a wrong assumption.  Why didn't you just ask her about it before picking them?  In your last sentence, though, you may have answered this when you imply she wasn't someone you wanted.  These are 2 very different things.  

I'm not sure what to tell you, but I disagree with anyone saying you MUST pick a sibling.  Yes, most of us do this for a variety of reasons, but it's simply untrue that there is anyone who has to be included.  This has never been true.

I'm shocked you didn't think of your sister in the first place.  She should be upset with you.

A big shout out to TJ and Common Sense, nailed it. Are you just plain dumb or don't give a rip?  Never make an enemy if you can help especially a family member and never hurt a family member.  Which of these people would donate an organ for you if you needed a transplant of some kind.  Family is family and you TREASURE them.  I would never do that to my sister.  You are all tied up in friends/sinlaws because of age; but, your sister should be your MOH. There is no law that says you have to have family or that she has the right to be upset; but, most people with just a lick of common sense know that this a time for family. Obviously, much to your suprising, you are shallow, petty and vain and only care about yourself.  Well, when everyone else is married and have children, move away, who's left???? Yes, it's YOUR wedding, enjoy it; you may never see your sister again if she comes. 9 years is nothing.  Grow up. P.S. You will look like an axx when people see that sis is not included in wedding in any way.

Say to her what you've written here, "it's my wedding and I get to choose the bridesmaids I want". Just don't be surprised if this forever undermines your relationship with her. It's curious as to why you'd be traditional enough to have a bridal party but not traditional enough to know who's supposed to be in it by custom. 

Well she has the right to feel what ever way she feels.

No I would not go into you get to choose who ever you want to and you don’t want her. 

Yes you do get to pick your BMs.  But you don’t need to make that a point 

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If your wedding party is set and you still feel like you do not want to add her in the wedding party? Then apologize and tell her you are sorry she is upset. 

Do you want to include her in anything? 

Do a nice lunch and go shopping for a dress for her to wear to the wedding. Ask her to help you with getting the cake or another aspect  of the wedding,

 I do not feel anyone is obligated to be asked to be in the wedding party.  Sisters are not automatically a maid of honor or a bridesmaid.  Not everyone is close to their sister and there is no reason why every sibling needs to be in the wedding party.

But you do need to talk to your sister

If my brother got married and he chose brothers in law as groomsmen and not me I would be furious, yes your sister has every right to be upset and if she skipped your wedding I wouldn't blame her. 

You cannot dictate on how you think a person should or should not feel. That's ridiculous.

She is upset and you are just downplaying and basically dismissing her feelings with zero empathy or realization that she is hurt.

Yes, your sister has every right in the world to have emotions.

What the hell is wrong with you?

yes she has a right to be upset. you chose your friends and boyfriend's sister instead of her.... it's weird. and saying "i'd rather have them by my side" is a little rude.

No-one, related or otherwise, is entitled to be a bridesmaid (or best man / groomsman). 

You 100% get to pick the women you're closest to; and that doesn't have to include your sister.