Aarti isn't to blame, she is a victim of her own convictions. She is at heart an honest person, in addition to that she is fiercely loyal to those she loves, and she loves deeply. The Dubey's are her family and she is incredibly loyal to them. Does that make her foolish? Yes, but most of us are when it comes to family.
The Dubey's have not dealt honorably with her or the S family. See this is the problem with lying, there's no such thing as a small lie. They intentionally set out to deceive both Aarti and the S family by saying that Aarti was a widow. Now lets follow the trail...
Originally they didn't want Aarti to know that she was marrying as a widow... when she found out and wanted to tell the truth they stopped her using guilt and debt ( at best an uncomfortable combination and at it's worse a deadly one).
Sr. Dubey has continuously lied ( by omission) about his contact with Prashant to Shohba...which since he didn't say that Prashant was not only alone but ill, left no room for Shobha to become sympathetic to Prashant's plight before he entered such an advanced stage of cancer
Then the Shobha doesn't tell Aarti about Prashant when she first finds out because of the pregnancy complications...good thought but at some point you gotta buck-up, but she didn't.
Sr. Dubey tries to forcibly take Ansh to the hospital ( for what I don't know) without Aarti or Yash knowing only to be stopped by Prashant (his only good act as far as I can tell and no I didn't forget he saved Aarti when she passed out in the woods). And Aarti finds out that Prashant is still around and has had contact with her son ( you know the one he's not sure is his). Instead of telling the truth they again do what... TELL A LIE, and so on and so forth until here we are...
Their lie, their mess, her fault. A day ago they were begging Aarti to tell Yash and she did ( we're led to believe he didn't hear her, but personally I still have a little doubt). Then they corner her after they left her son with an ill Prashant who proceeded to promptly pass out. Aarti took him to the hospital leaving her husband and family to do the needful. She left a gift from her mother-in-law to get medications for Prashant and she had no idea what was wrong with him until the pharmacist told her.
I understand why the Dubey's have done a lot of what they have done but their messy behavior is their own. That's how they choose to handle things, to apply familial love as an ax to the neck is absolutely unjust and that's what they have done to the most loyal member of their happy little band of thieves. And all this started from one little lie.
***NOTE***
I didn't assign blame because at this point everyone involved with the exception of Ansh (and since he lied to his Mom he's really skating the edge here) has contributed to where we are now.