Can definitely understand why you feel left out.
On the other hand, it very likely isn't to do with you, but rather they have maybe just 'clicked' and formed a tight friendship. That doesn't mean that they don't value you, just that they are closer still. Sometimes that happens, where two people will get along on a deeper level - it has nothing to do with family or leaving others out or whatever, it's just a personality 'click'. IYKWIM. The probably believe they are including you, by having you there as much as they do, not excluding you by meeting up without you.