Thinking about all 45,000 weddings I’ve been to I feel like you’re right. If the groom has a sister it’s fairly standard that she’s in the bridal party and even more so if it’s on the larger side. So I can totally see why not being included reads like a snub.
Buuuuut also? Money wise she did you a huge favor. No stupid dress you didn’t pick out. Do you get out of going to the bachlorette party? That’ll usually set you back a couple hundred dollars.
She could legit be one of those people who’s like *your family* and *my family* are not the same thing (in the marriage sense). One of my good friends is like that about her husband’s family and the rest of us sometimes are like “dude, that’s your kids grandma be a decent person here” and she thinks we’re all insane? So if that’s what’s happening it’s not personal, it’s just how she is.