Feeling left out and ostracized is the story of my life. I know this feeling well. I'm a little strange though. I wasn't born in this country and was raised in a very different culture. I've lived in the US since I was 7 years old but even today things will come up in conversation and I have no idea what that is or what anyone is talking about. I'm pretty blunt and have an extremely dry sense of humor. Basically, I can come across as a real b*tch. As much as I like to think I've accepted this about myself I still get my feelings hurt when I get left out.
Once in high school a group of us were planning a day trip to a big waterpark (Schlitterbahn) and we spend a couple of weeks planning the whole thing. I grew up super poor, but my dad came up with the money for my sister and me to go on this trip. The morning we were supposed to go we waited and waited for them to come pick us up and they never did. Turns out the group didn't think we'd have our money and just went without us. It still makes me mad to this day.
That story notwithstanding, most people have no intention of hurting your feelings or making you feel left out and I'm sure that is not their intention. They may have been planning to discuss something that they thought would be boring to you or they may not have even thought that you'd care to be invited.
I usually make friends ok but have a hard time keeping people really close....well women anyways, because I take more of a man's approach to friendship. I dont like talking on the phone much, and as much as I am touchy feely (like hugging and stuff) and can and do empathize, I just get really irritated with needy friends. You know the ones who almost feel like you're dating them! I have a friend right now that gets all "worried" when I dont reply to her text right away. She's paranoid i'm "cheating" on her with another friend maybe? I dont know.
But I would feel snubbed by that situation too. Especially if she said you were all getting together. But I'd probably get over it faster due to my approach above. I'd just figure "huh, well she's not worth it, time to move on."
I think your sister does have a good point, but yeah, I would feel hurt in that situation too. Although I am kind of the friend who never stays in touch, I don't like to talk on the phone so I don't usually call. However my friends are all the same way so I guess we fit together well personality wise. We will send an email or text every few months and when I am visiting the town they live in, I will usually call and let them know I am in town so we can get together.