you are a bit crazy jealous with a point
there isn't anything going on between them and certainly if there is it is NOT coming from your bf. if anything they may have that "female competitive" thing going on with you and this is more of a battle of supremecy over his attention (but not his love and affection) between them and you. (Q: is it a specific gal of the 2 that keeps doing this.. or is it both of them? aka is there a ringleader in these antics or no?)
NOW.. my recommendation here is this: talk to your bf and say that this has to stop. You are NOT dictating what he does with them or his friendship with them (unless of course it goes romantic.. THEN its' yoru business) - but when it's 'WE TIME" together for you two.. it's WE TIME. It is up to HIM to manage his friendship with these two and set appropriate boundaries when it's YOUR time together. (of course.. a good gf always allows for some shanningans and lets it go when it's OK...) But purposely interrupting you two during what is usually private time (and yes they are purposely doing this to a degree) will NOT be accepted.
If he can not or it is not a priorioty for him to protect your relationsiph and your time together - well then.. you will have to consider that as part of how he views your relationsihp and its priority to him.
If he doesn't shape up. walk.
if he does... there you have it.
- - - Updated - - -
btw although it is an intrusion issue.. let's be honest.. it is a jealousy issue too. whether you realize it or not.
this is a typical female "territorial" pecking order battle going on.
"no matter what and who you are he will pay attention to me instead of you" type behavior and staking out territory with your bf.
you may not acknowledge that but deep down you know its true (and with them... and they are challegning you b/c you're the queen of his perch right now). so the more genuine, honest, and self-acknowledging you are during these discussions.. the better.