Ok, I feel like James here, but we don't know the full story.
I DO NOT KNOW JESS-POM SO I AM NOT SAYING THIS IS THE CASE *BUT* could it possibly be that the bride had a perfectly valid reason for not wanting her along? Not everyone gets along with their sister you know. For example, what *if* jess-pom likes to say rude things or doesn't like the groom and "makes it very clear ha ha." We just don't know.
That said, if everything is hunky dory and there's no reason why jess-pom going along should be objectionable, then I don't see why her sister wouldn't let her go. Then again, some bridal shops are weird and they only allow so many people in at one time (to prevent the bride, her mother, mother-in-law, 5 sisters and 6 bridesmaids from clogging up the shop floor). Who knows?
It's only my opinion of course, but perhaps jess-pom, you might like to sit down with your sister and politely tell her how you feel. Tell her how much you are looking forward to being a part of this special time for her. Tell her you were very hurt when you were told you couldn't go along dress shopping. Ask her to explain why you "weren't allowed" to go too. Ask if there's is perhaps another way you can help out.
Good luck; I hope it works out for you!
.....Kelly