Whether an adopted adult child can contest the Will of their birth parents, will depend on the circumstances surrounding their adoption including the age they were adopted.
As a general rule when a person makes a new Will, it will contain a clause revoking all previous Wills that they have made. This is commonly referred to as a revocation clause. If you have two valid Wills signed on different dates, that do not contain a revocation clause the Court will deal with your Wills, in accordance with the principles of implied revocation. More
What is a Defacto Relationship and are there any precautions to take or any legal papers to sign? More