Good question. If she just wants to come to the U.S. to study she does not need a U.S. Citizen or resident sponsor. She would need to apply for an F-1 student visa. Most U.S. universities have international student offices that can assist her with this process. You could help her by assisting her find these resources. If she is interested in coming to the U.S. for a non-university education (i.e. vocational school or secondary school) there are also options available for her. In either case, there is not anything you can do to help "sponsor" her but you can help by connecting her with the appropriate resources. The first step would be to identify the U.S. educational program she is interested in and then contact that institution about some assistance. If it is a secondary school you may not find anyone who can help at the school and may need some help from an immigration attorney. Hope this helps.
If you are asking about family-based sponsorship for permanent residence, then I'm afraid not on the basis or relationship alone.
Your son-in-law's sister (I am assuming for our purposes that she is over 21 years of age - if younger, the answer may be different) can be sponsored by a citizen or permanent resident spouse, a citizen parent, a permanent resident parent if she is still unmarried, or a citizen sibling (your son-in-law, if he is or becomes a citizen, can sponsor her).
Now, this does not mean you could not ssponsor her on the basis of employment if the circumstances fit. And of course, as the attorney who previously responded wisely pointed out, there may be nonimmigrant visa options to bring her here in the interim.
As always, I recommend speaking with a qualified immigration lawyer. There are many, many variables here that must be addressed, and I've made certain assumptions in responding which may or may not be the case (her age, lack of factors which might prevent her getting permanent residence which are completely unexplored here).