Unfortunately one cannot contest a death certificate per se. What can be done is to contest any subsequent proceeding by your sister's former boyfriend claiming that he was married to her so as to make a claim on any assets that she may have. If your sister passed away with a Will or a Trust and he is named in it, then there is little that you can do about the former boyfriend making a claim under the two documents assuming they are valid and properly created and executed.If there was no Will or trust for your deceased sister, I would simply take a wait and see position concerning the former boyfriend to see what claims he may be making to your sister's estate and respond accordingly preferably through a Wills and trust attorney. If he is claiming to have been married to your sister, he will have to provide a certified copy of the marriage certificate.