What is the ruling on gifting a car to someone who is most likely to use it sometimes for unlawful purposes, or building a kitchen for someone who is most likely to use it sometimes for unlawful purposes, based on the Sharia principle that prohibits a gift if it is highly probable that it will be used for unlawful purposes?

It is permissible to gift a car to your relative, as long as its primary use is for permissible things. Their driving it to amusement parks or beaches is not necessarily forbidden. And even if it were to be used sometimes for forbidden things, this would not prevent the gift, because the ruling is based on the predominant use. Aiding in something forbidden, if it is not intended and not direct, is not forbidden. One should be wary of scrupulosity and undue strictness.