Can I sponsor my mother in law to come to the United States? I am a citizen, but my spouse is not.

When people talk about sponsoring someone to come to the U.S., usually they mean petitioning for that person's green card. You cannot sponsor your mother-in-law. You wife can petition for her mother, but the process will take several years - unless your wife becomes a U.S. citizen. If, however, all you want is for your mother-in-law to come and visit with you for up to 6 months, the situation is much simpler. One does not need a relative's petition to obtain a visitor's visa (B2). However, if you write a letter to the U.S. Consul in Hong Kong explaining that your mother-in-law would like to come only for a short stay, has no intention of remaining in the U.S., and that you have the means of supporting her here (attach your most recent federal tax return and a current letter confirming your employment, with 3-4 recent pay stubs), then her chances of obtaining a B2 visa will increase.