If there's one thing I'm learning in planning my wedding right now, it's this:

In every single step of wedding planning, you're going to hurt someone's feelings.

Pick a date? It doesn't work for Future MIL's sister.
Choose a hall? It's too far away for Dad's cousin.
Picking food? Someone's allergic
Going dress shopping? Someone's hurt they didn't get invited.

The moral of my story here is, I understand that you're hurt. I don't have any sisters, but if I didn't get invited to go with them, I probably would be hurt too. But please try to understand that your sister is in an impossible place. She's trying to please everyone and she simply can't. I'm very, very, very sure that she didn't mean to hurt your feelings or make you feel excluded. She's probably just so overwhelmed that she doesn't know what to do next.

Tell her that you're sad you didn't get invited, but that you hope that she'll let you help her do xyz (address invitations, drive her to a fitting, whatever). She'll probably appreciate the offer and be glad to know that you're there for her.

*hugs*