@Tulip1011 YANBU to be hurt and upset, but you won't get THAT answer from many on mumsnet. Most people on here don't let ANYthing get to them! They don't care if they get no birthday cards, or if they don't get invited to something that their partner does, or if their husband goes away with a woman on a weekend trip, and shares the bed with her, or if someone who is a good friend of many years prioritises a friend she met last month over them.
And all the 'what are you 6 years old' and 'you sound like hard work,' and ''you sound immature, pathetic, and childish' comments are so rude and unnecessary. 
Just ignore the vitriol and spite on here OP.
No real advice though sorry. Other than just give them all a wide berth. No doubt their amazing chummy friendship will run its course, and if they're lucky, you may be around to listen to their woes and their bitching when they fall out, and they need you again. And it will happen!
On the other hand, you may have moved on.
Bit awkward with it being your sister though. Would be easier to avoid and ghost if it was just a mate.
Good luck. Don't let them get you down. 
As I say, just give them a wide berth. Don't contact them, and don't get in touch with them. Let them run after you!