The very fact that she felt that Yash might refuse the BMT if he knew the truth proves that Aarti doesn't trust him completely, inspite of the fact that he was willing to give up his life for a stranger. Aarti should have known that no matter how hurt or angry Yash was, he would never refuse to save Prashant's life!
Also, u say that we simply can't blame aarti for the lie, really? u say she tried to tell and he fell aslep? Kindly tell me when was that? after about 6 months of their wedding! Why did she choose to listen to Shobha and hide the lie when she had decided to tell him after the shootout? Shobha did not emotionally blackmail her then, it was Aarti's decision too. We can't just blame Shobha for it! Why didn't she have the conviction to tell the truth? she was not in love with him yet, she was scared for Ansh, right? So what does that mean? She chose to keep Yash and his entire family in dark so that Ansh could have a father...where does Yash come in all this? I still am willing to overlook that, because at that moment, she had to prove herself to the Scindhias, and more importantly to Palak and Payal...so she needed time. But after that? what happened in Mumbai? When she met her friend? She ran away with Yash from the place, and never felt the need to tell him the truth even then! For me, hiding the truth inspite of meeting Sunita in Mumbai that day was her greatest mistake! She was not under any emotional blackmail then, it was her fault. Yes, she was scared of losing him, but then that really does not give her the right to lie! While I can sympathize with her, I certainly can't say that she simply can't be blamed for lying to Yash!
However, what she did regarding the BMT, thats the worst of the lot! She took away Yash's right over Ansh, though she herself does not think so, but that is what it would look like when it comes in front of Yash! After Yash had fallen asleep, she had got several chances to tell the truth, even before she met Prashant, but she didn't. Even when Yash overheard part of their conversation, and Shobha was lying to cover up, aarti's actions were also how to cover it up,. She had not been told about the BMT yet, so why was she covering it up then? I would have been happy if she had attempted to tell the truth, and wouldn't have minded if Shobha stopped her somehow.but she made no effort. Yash had asked her a question, and she stood there.silent, her expressions indicating she was busy covering it all up, and then Shobha lied to divert Yash. Is that not her fault too?