Up until The only risk of someone finding out infidelities, sealed adoptions and such was if someone talked. These days it is a different story altogether. With massive DNA databases and billions of people contributing to it everyday people are finding out things about their family that were secret. Siblings do not even have to do a direct paternity test now to find out if they share the same father or rather do not share the same father or that they have other siblings they never even knew about.
If doing direct paternity testing is not an option then the grandparents can test themselves with the grandchild in a grandparent DNA test. Since the child inherits 50% of his or her DNA from each parent then they would inherit 25% from each grandparent. Because there is less DNA to Since the child inherits 50% of his or her DNA from each parent then they would inherit 25% from each grandparent.
Not all states favor illegitimate children so estate claims may be different from state to state and even if you are the biological child if you are illegitimate it may not matter. The best advice is to contact an attorney in the state where the father's estate is located and find out what the options are first.
Non-consensual DNA testing can also be an alleged potential father taking a mouth swab from the child without the child's mother's consent or the childs consent.
So far there is no Federal DNA theft law in Australia or the USA but other countries such as the UK have enacted laws prohibiting testing another persons DNA without their explicit consent or authorization.