I’m sorry that you’re unhappy about your future SIL’s behavior.
From a money perspective, think of all the time and money you’ll save by NOT being a bridesmaid! (Also I would be annoyed to have to do/pay all that for someone who isn’t really interested in including me in the group, but only doing so out of pressure from her fiance.)
I know how easy it is to take things personally, but statistically it is likely that she just didn’t think to include you as a bridesmaid. I wonder if she has 7 very close friends and figured that adding more bridesmaids was overkill?
I’m not sure you can really have any expectations on your SIL, even once she is married to your brother. Obviously, she should treat you with respect, but I don’t think she has an obligation to be your friend or give you a special role in the wedding. This may make her seem like a jerk (she might be a jerk) but it might just be that she is slow to warm up to people, overwhelmed with the labor/emotional labor of planning a wedding. It’s probably easiest to work on adjusting your expectations and just doing your part to be the kind of SIL you’d want to have.