Guinea pig defensive jump?

I have 2 Guineas, one (called Jordy) is an absoulte darling who loves attention and is quite happy for you two pick her up and pretty much makes my life easy, the other (called Connie) really doesn't seem to like me or Jordy, she's never bitten me (although I wouldn't put it past her) but she bullies Jordy, she runs rampant around the cage, jumping in and out of it (they free run regularly) attacks Jordy, when I try to pick her up she speeds off and when I do manage to get her she does this defensive jump/kick thing and squeaks at me, she's not injured, already had her checked for that much to her disgust, they are still quite young and they are from the same litter, the woman at the petshop said it's because she's trying to fight for position of 'master guinea' which apparently I should be, when I asked what's the best way to win the position back she just said 'dunno' helpful, any ideas? Is the pet shop lady right? X