My MS advisor who is a tenured professor in a US school is padding his wife's name into my publication papers. His wife is an PhD candidate from the same department too and she didn't have any contribution on my research work. This is ethically and morally wrong but as the advisor is my thesis committee chair and I want to do my defense in coming month, I haven't opposed his idea yet. What I could do in this scenario? (He submitted one paper in a conference already with her name as second author and I am working to submit another in a transaction- he insisted her name to put in the second author position)
This sort of "guest" authorship is not unheard of. For instance, that may be why Chinese universities primarily give credit to the first or corresponding authors of papers (as opposed to other authors, who may be "guest" authors).
Assuming your description of the situation is correct, including another PhD student who had little to no contribution as the second author, especially one with a personal connection to the advisor, is definitely ethically questionable. It's good that you noticed and disagree with it.
Practically speaking, it's much easier to raise this issue after you have graduated and received any recommendation letters you need from your advisor. For instance, if indeed you are submitting in a few months, you could delay submission of the papers until afterwards. Alternatively, even if you submit a paper with a non-contributing second author, as the first author you will still receive the primary credit from any applications of your CV.
The situation you describe is known as the "White Bull effect" and was for example described here:
"Junior researchers can be abused and bullied by unscrupulous senior collaborators. This article describes the profile of a type of serial abuser, the White Bull, who uses his academic seniority to distort authorship credit and who disguises his parasitism with carefully premeditated deception. Further research into the personality traits of such perpetrators is warranted."
what you can do? In your current situation: Most likely nothing. You can hardly win anything but lose a lot. But on the brighter side, in most fields only the first and the senior authorship count anyway (for exactly the reason described in the paper above).