My friends are leaving me out?!?

So we have five people in our "group" I'm the oldest (8th grade) my best friend is the same age but in 7th grade, her little sister is in 5th grade and we have two neighbors who are both in 6th grade. Every week we go up to the "Farm", we go snowboarding and tubing, and explore the creeks and the woods. But for the last couple weeks they haven't invited me. They've lied about going, and are ignoring my texts and calls. I know my best friend wouldn't exclude me, we've been best friends since pre-school, but she's kind of special needs... Her little sister kind of manipulates her into doing stuff... Every time I see them outside I go and meet them, then they make excuses and leave... Then 5 minutes later they meet back up again! I've been trying to talk to them but they won't call/text me back, when I go to their house they pretend that they are not there, and they refuse to make eye contact with me on the bus! I don't know what to do! I don't know what I did to upset them... Whatever I did I can't apologize for becaus they won't even talk to me! Please help! I miss hanging out with them and laughing with them... I know they obviously don't want to be friends with me anymore, but I'd still like some closure... I just need advice!