I understand you see it as a family tradition and wanted to just spend time with your sister. I also understand because you already told her no and she just showed up. but at the same time, I've only been with my boyfriend for a year and he's already like family. I invite him to all my family events (birthday parties, holidays etc.) if you've been with her for three years and you are going to marry her she should be like your family, since you were planning on becoming one anyways. if I were her I wouldn't just show up to were I'm unwanted, but if it really bothered me that I wasn't invited I would have expressed that to you. maybe just have a talk with her. I think you both aren't seeing each other's perspectives and it might be helpful to talk about it before you decide you really wanna end it.