My close friends hangs out more with my older sister than me?

Why is this? I don't understand. (I'm 17 she is 18, my sister is 25) Like me an Alexandra would see each other every day at school and do nothing hit talk and talk. Like we are pretty good friends! She graduated a week ago. But every time my sister comes home for the weekend she always wants to go places with her, go get food late at night for fun, go get snow cones but Alexandra never invited me. Like she will just ask my sister if she wants to go. Why? Like we're good friends probably closer than she is with my older sister. And they text allllll the time. Why doesn't she treat me the same?