Is it permissible to sell a plot of land and relinquish it to a maternal half-brother so that he may falsely swear to its ownership and sell it, in order to avoid tribal threats that prevent the family from disposing of its properties?

There is no harm in you ceding the land to your brother for him to sell it on your behalf, and there is no harm in the buyer swearing that he bought the land from your brother, because your brother is your agent, and the agent stands in the place of the owner. There is also no harm in him swearing that the land is his property, and his oath would be from the perspective of ma'areedh (allusions) and tawriyah (equivocation), or swearing a false oath to extract a right from an oppressor, and this is permissible.