Which event should I attend?

My half sister's other half sister is getting married on November 21. I spent a small part of my childhood (as in maybe a year when I was 5 and she was 3) with this girl, but haven't kept in touch for years upon years. I barely know her in all reality. However, for some reason I was invited and my half sister expects me to go. Well at the exact same time and day we are having our family's Thanksgiving dinner. I happen to be pregnant and am due in March, so I umed my aunts and cousins would be wanting to talk about having my baby shower (which would presumably be in January) while we are all there to synchronize schedules. This is my first baby, so we really need to have the baby shower planned because we can't afford all these new things. And this is also first baby to be born in our family for almost 10 years, so everyone is very excited about seeing me pregnant and planning a baby shower. Our family rarely gets together and I think its important to visit with them for Thanksgiving rather than going to my half sister's half sister's wedding. But I don't know if that is the right thing to do. I mean she only gets married once. But then again, I'm not sure how many other babies I will have either, this may be it for me. I don't want to be selfish, but I feel torn in whether I should sacrifice for a girl I barely know, who probably won't care or notice if I don't show. I need some advice.