Being ignored by your own family member?

I have 3 cousins. 2 girls (let's call C1 and C2) and a boy (C3 then). I'm 14 and they are 16. Me, C2, C3 live in a city, while C1 lives in a city that is very far away. But me and C1 chat a lot with each other, and when she visits me or I visit her, we usually do many many stuffs together. I even taught her how to swim. We have been close friends more than cousins for a long time. C3, however, isn't as close to C1 as I am. But that doesn't bother me much. Me and C1 share a lot of hobbies, such as watching movies, reading novels, and a lot more. In this summer vacation, C1 came to visit me, and it has been 4 days since she landed in my city. In the first 2 days, things were quite well between us. But then, in the 3rd day, things started to got worse. C2 appeared. As first, I don't think that she is a threat or something. However, C2 and C1 found some common hobbies: they like to listen to K-pop and I don't, they like to listen to Justin Bieber, while I don't,... It started to get worse when C2 started to sleep at C1's house (as I said before, C2 was on a vacation to the city where me, C1 and C3 live), instead of my house like before. Now, every time we hangout, C2 only talk to C1, and they practically ignored me and C3. At least that what I thought. I didn't talk to them about this. Please help me, I feel desperate as I only have 3 days left to spend with C2.