score:1
Your mother's application for a US visa was predicated upon a visit to her daughter, husband, and child. That purpose has now disappeared, and as Traveller has noted the other Answer, the consular official who'll conduct the interview very likely knows that the relatives' visas have expired, and that they should have left the US by the time your mother arrives.
No matter what your mother says when confronted with this, it will go badly for her. It will not be convincing if she says she didn't know about the visa ending, because all possible causes (your sister never told her, or mother's not telling the truth in the interview) reflect badly on your mother's and your sister's credibility. If, on the other hand, your mother admits she knows about the visa ending, or even that your sister and her family have already left the US, your mother will be admitting that the reason and justification for the visa have now disappeared. In either event, the visa will be denied and your mother will have a black mark against her credibility, making any future visa application less likely to succeed.
Worse than this is your mother's failure to mention in her visa application other relatives in the US. This too undermines her credibility, and there will be little persuasiveness in her responding "I didn't remember them" or "I didn't think they'd matter here."
I don't think there's any benefit to her in showing up for the interview, as anything she says will make the situation worse. She should consider either failing to appear for the visa appointment, or endeavoring to cancel it. If she doesn't show, she'll forfeit the application fee, but she won't dig the hole she's in any deeper.
If she ever wishes to secure a US visa in the future, she should consult a US immigration law attorney not an "immigration consultant" in her country about how to address the didn't-mention-the-other-relatives situation in this visa application.
Upvote:1
If your sister and her family will no longer be in the US when your mother travels, what will your mother say when asked about the purpose of the trip and who she intends to visit? And is then asked why she answered βnoβ to the question about other relatives? (Why did she do that, by the way?)
As explained here https://de.usembassy.gov/interview-and-supporting-documents/ the interview is an important part of the process. It might be wise to assume that the Consular will already know from background checks that their visa has expired, so a refusal is possibly already on the cards. But no-one can guess the outcome; whether your mother goes ahead with the appointment or cancels her application is up to her.