HELP! I think our friendship group is falling apart...?

So basically I'm best friends with these girls Olivia, Kaitlin and Abbey. Abbey has always stuck by me and been an amazing friend. But Olivia is going off with my worst enemy (traitor) and Kaitlin is hanging out with older girls. So are they not true friends? Shall I let them go? Apart from Abbey because she is a great friend. Thanks.