Citing abstract only papers/conference proceedings I'm doing my MS in Astronomy and Astrophysics so almost all of the literature I need is readily available on NASA ADS website (ui.adsabs.harvard.edu) with links to downloadable PDFs and HTML articles. But sometimes I come across conference proceedings, or some old articles where only abstract is available for reading but it DOES contain what I'm looking for.

Since I can't read the whole thing, is it okay to cite that?

Apart from the original links to downloadable PDFs, if something I want to cite is in the abstract but I want to dig deeper and the paper is not available for download in any form, should I look for other papers that have same information and are available or can I use sci-hub. And how fair is it to use sci-hub?
It is a bit risky to cite papers having seen only the abstract. You might be able to contact the original authors in some cases for more complete versions. 

But, the safe and approved way to get access to papers that aren't easily available online, or available only by conflicting with with ethical considerations, is to go to your local library and ask the librarian there to get you a copy. Academic/university libraries are best, of course, but most libraries have some sort of access, directly or indirectly to nearly everything. 

Moreover, a research librarian at a good library is a tremendous resource for finding obscure things and even related things you don't know exist.