It is not inherently sinful to contest a will. What could be sinful are the intentions for doing so. If someone was contesting a will because of greed, envy, revenge, etc., then that would indeed be sinful. If, however, someone contests a will because it is truly unjust, that would not be sinful.
Raphael Benedict is a Catholic who wants nothing but to spread the catholic faith to reach the ends of the world. Make this possible by always sharing any article or prayers posted on your social media platforms. Remain blessed