Q: My sister's husband isn't Jewish. Can they both have an aliyah?
A: Rabbis across the spectrum, from Orthodox to Reform, are largely in agreement that non-Jews can't have an aliyah. After all, an aliyah is a prayer, recited on the stage (or bima, in Hebrew) that is used to thank God for choosing "us" and giving "us" the Torah.
You can always ask your rabbi how he feels about calling up your sister to the bima and simply having her husband accompany her but not sing the prayer. On the other hand, there are many other wonderful and inclusive ways to honor your sister and her husband simultaneously that involve participating in other aspects of the Torah service. You can ask them to, say, open or close the ark. Or recite a meaningful poem. This will certainly make your brother-in-law more of an honored participant in the ceremony.
You should know that there are a few radical voices out there arguing that there is one time that non-Jews should be allowed to have aliyot: when that person is the non-Jewish parent of B Mitzvah child. As Edmund Case, publisher of InterfaithFamily.com and co-editor of The Guide to Jewish Interfaith Family Life: An InterfaithFamily.com Handbook, has written: "an intermarried non-Jew who has participated in raising a child as a Jew to the point of that child becoming bar or bat mitzvah could say, with complete integrity and authenticity, that his or her family is included among the 'us' who were chosen and to whom the Torah was given. Moreover, such a parent arguably deserves the highest honor that the Jewish community can bestow.