wait- you're asking if jamaicans are going to be racist or racially insensitive to your african sister in law and not if white tourists are going to be racist or racially insensitive to your sister in law?
i could go into a whole thing about how i've only been to jamaica with white people twice in 25 years and those are the only 2 times i've ever had any kind of awkward interactions, but i'm gonna keep it cute and say that jamaicans love when black tourists (especially black tourists from africa or other prts of the caribbean) spend time in ja.
my husband and son are from port au prince- my husband is SUPER haitian, and, after they figure out he's actually not jamaican, people are FASCINATED by the fact that he's from haiti (and not a drug dealer or a rich, white haitian, but again- i'm gonna keep it cute and leave it at that). i'm sure your sister-in-law will get the same warm welcome, especially since she's african.