Not sure how to feel

I am so sorry, I know both of those things can be really upsetting. It's hard, but take a moment and put yourself in her shoes. Without more context, it's hard to know, these both could be a little rude, but not huge deals, in some cases.
I agree with a previous poster: If she is having a small small wedding, I would not over react to your SO not being invited. That being said, it is still rude. I am having a 55 person wedding and anyone with a steady SO is invited, even though its just our (large) immediate family and a small amount of our closest friends. But 10, 15 people...more understandable. But if it's normal sized, seems like she knows what she's doing, you have every right to be confused and upset she has not invited a SO you have been dating that long.
As for the not being a bridesmaid...this is a hard decision for any woman. Ask yourself: Is this coming as a surprise because you expected her to choose you because you are sisters, or is it because you are close? When you look at how close you are with her and how close she is with your other sister, are you confused because you have the same closeness, and don't understand why she has done this? Or are you upset because you feel they are closer and this feels isolating and is solidifying those feelings?
If you are close with her and that is why this is a surprise, my advice is to just talk with her. Say, "hey, I'm not mad, but I just need some closure with this so I can enjoy your day. I just wanted to know if there is something behind the scenes I don't know about, or if there is something I have done that has made it so I'm not a bridesmaid, I was really expecting to be one. It's truly alright if it is just that you had friends you are closer to, I just want to make sure I haven't done something." This is NOT a conversation I would have if you're thinking "Wow, we are definitely not close enough to have that conversation civilly." Because if you aren't, that's probably your answer right there: you maybe aren't close enough to be a bridesmaid.
If you only expected to be in it because you are sisters, and you are sad because you feel like this is pulling you farther apart...I'm so sorry. Your feelings are truly valid, but unfortunately, I wouldn't bring it up with her. Weddings can cause some hurt, and I'm sure she was not trying to hurt you directly with this decision, but she probably took a long time to make this decision, and truly considered you and your feelings. She could be broken up about it as well but needed to choose a close friend, or she could not even think you are hurting because she doesn't think you'd care about it.
I wouldn't not attend because you aren't a bridesmaid, but if she is having a normal size wedding, I think it is pretty pointed of her to not invite your SO, I don't think you're obligated to go.

Well... I am 1 of 6... One of my sisters asked me whose side she was standing on (she was a butch lesbian so definitely wouldn't wear a dress) I wasn't planning on having any of my 4 sisters in my bridal party... this is one of my biggest regrets. This same sister committed suicide back in January(unrelated to this, or maybe this had a small factor but there was a lot of other stuff going on mentally)... After her passing I got very close with my siblings (we have all always been close but there was a bigger age difference between me and all of my other siblings, my sister that passed was closest to me because we were close in age). I ended up asking 1 of my other sisters to be in my bridal party, but not my other two (they are married and I wasn't in theirs and they are in their 40's). It sucks but honestly my sisters didn't get offended I didn't ask them even though I did include my other sister. Sometimes its an age thing, proximity theres lots of factors. She could be doing a small bridal party and while it sucks it is something you will have to let go.. the significant other though... Thats inexcusable. You have been in a committed relationship and she knew about him. Thats just not right and I don't blame you for not wanting to go for that reason. Shoot... I have guests that have been dating people for a few months and I fully expected them to bring them. I gave everyone a plus one (even if they weren't dating someone).

Perhaps your sister did not make you a bridesmaid for financial reasons, or maybe she feels that you guys aren't close, and decided to choose the sibling she felt closer to. For sure, that would sting though. I am just throwing ideas out there. But.. for her to tell you that you can't bring your partner is absolutely disrespectful. Why should I share in anyone's union if they aren't going to simply acknowledge mine? I would really ask her why did she not invite him... If someone told me I couldn't bring my FH to their wedding, I would be so upset, and would not attend the wedding.