Well, the only positive thing is that he did introduce you to his parents and siblings. I guess even the mom knows how psycho the other girl is but her son just can't get rid of his addiction. But it doesn't matter how highly his family think of you. He should be treating you like a queen. You wrote that he will be going to Las Vegas without you!? Who cares if he is going alone or with HER, but if truly loves you, of course he wouldn't leave now when there are problems in the relationship to be worked out, and better yet, he would take you to Las Vegas. Who would want to hang out with a bunch of cousins over a girlfriend!??
I don't understand why he always has the nerve to tell you that she will be here and there! His biggest problem is that he can't end it with HER and affecting your relationship. You asked what if your assumption is wrong.... well, first of all, he said he will end contacts with her for AWHILE, but then what happened!?? He will go to Las Vegas with her included! What is that!?!? Then, he is going to Las Vegas instead of working out problems between you and him. He's just walking away like he doesn't care or don't want to deal with them. He said he loves you, but where's the action??? Everything he does shows otherwise! I even suspect if his cousins are really going, and if they are, do they have their own room away from the two of them!
Does she live in the same city as him and you? If not, you can take a look at his charge cards or receipts of gas stations, dining, department stores in her area. That's what I did. I also saw telephone bills, both home and cellular with her numbers and calls that lasted 30 minutes, 45 minutes, etc... But in your case, he's actually telling you that SHE is in his life all the time. Just remember, he has the major problem of not getting rid of her, she's not the main problem. If he truly loves you, even if she calls him up and wants to see him, HE can refuse her.
I still have the other girl's phone number, but I don't want to call her anymore asking if they are still together because I accepted that it's my boyfriend who has the problem not her. Since I know I won't marry him or have him in my life for long term, I just deal with it until I have the strength one day to get rid of him for good. For now, I'll be the stupid one.
Oh, and asking him to be faithful, truthful and committed is not DICTATING his life. That's asking for a mutually respected and trusted relationship!
When he returns from the trip, don't even show him that you missed him! Force yourself to be happy even if you must. He's making you feel miserable and doesn't even stop it!