What is the ruling on laughing when imagining a female relative wearing the niqab, knowing that this is uncommon in society, and is this considered among the whispers of disbelief (wasawis al-kufr)?

One must completely and utterly turn away from misgivings (waswas). Dwelling on them leads to dire consequences. Among the things that help in getting rid of them are: seeking assistance from Allah, sincerely resorting to Him, and supplicating frequently.