I would be skeptical, too. I don't know that there is anything that can be done right now. Can you get your brother to postpone the wedding? I mean, if he loves her and was planning on marrying her before this, that is different. But if they weren't in that place yet and just because she is pg he thinks he should marry her, perhaps he should wait a little while.
I am not sure there is any way to prove her truthfulness except to wait and see how things progress. Offer support, don't let on that you are suspicious. In the end it may only serve to make you look bad. Hang back, step in when asked, and just quietly note the inconsistencies in her stories. Hopefully she is being truthful and that things are moving along okay. It is possible that one sac is growing faster than the other, something that can happen at any point during the pregnancy.
Boy, this is an odd situation. Let's see...
I can totally see your point about her saying she was 6 weeks, and now 2 weeks later she is still saying she's 6 weeks along. That does sound very untruthful. However, sometimes the doctors give you a different due date after an u/s. So maybe she got a different date during one of her 3 u/s and failed to fully explain why she is still only 6 weeks pg. Have you directly asked her about this discrepancy?
Also, being that it's your brother and his girlfriend, I'm not sure there is a whole lot you can DO. The best thing might be to gently explain to your brother what you know about the situation and express your uneasiness about her truthfulness. If he's not ready to jump into marriage then he should be willing to listen to you. But if he's not willing to listen then I'm not sure there's much you can do for him. You can also just tell her that you're there if she needs someone to talk to.
Also, you were wondering if the babies could be different ages. The answer is yes - but only if they are fraternal twins. Sometimes when a woman releases more than one egg, it's not at exactly the same time. They're usually within 24 hours of each other, so her saying they're two days apart might be what the doctors are saying to her. There is always a margin of error with u/s so they could very well be telling her this.
Now, with regards to what you can/can't see on the u/s: If she truly is only 6 weeks pg, then it's not entirely surprising that they can't see anything. Sometimes this does happen. I understand why you're questioning it - you'd think you could at least see SOMETHING, not just an empty sack, but at this early stage it's really difficult to say.
I'm sorry. I wish I could offer more advice. It does seem fishy, but at the same time the story could very well be true.
Is there a reason she would be faking a pregnancy? Have they been together a long time? Were they planning on getting married before she got pregnant, or does she want to get married because of the pregnancy? If she is pressuring him to get married because she's pregnant, I would definitely hold off. A baby does not a marriage make. Just to let you know, I think it does sound legit. My mom was a fraternal twin and her sister was a day older than her, gestationally. Came out at the same time, but not exactly the same age. Also, they couldn't see anything until I was 9 weeks along on the ultrasound, so the fact they couldn't find anything yet is a possiblity. They changed my due date as well, which would make you change how far along you say you are. Unless there is a possible reason for her faking it, I would assume she is telling the truth. But, now that she's pressuring him for marriage, I'd keep my eye on it.
They have been on the rocks lately. So I think she is pressuring him. Also she's had a total of 4 U/S already and only 6 or 7 weeks. Why? I only had 2 my entire pregnancy.
Ok wait, if she is only 6-7 weeks pregnant, that means she's probably only known for 2-3 weeks at the most that she's pregnant, and she's had over 4 u/s done? Is she high risk or something? I guess multiples could be a reason, but is that many u/s necessary? Why wouldn't she let you go to the doctor with her? What was her reason behind it? Has your brother went with her yet for any of those u/s? I can see why you are being a little skeptical, I might be too. I'm 34 weeks pregnant and I've only had 2 u/s, one at 9 weeks and another at 19 weeks. But I'm not high risk.
I only had 2 as well.
It's probably because of the twins, and the fact that they can't see anything yet. They're monitoring her closely, waiting to see something. The docs might be thinking blighted ovum too.