Death Certificate Being Challenged In Court

has passed away in 1998. The case is in progress where the defendant's lawyer claims that my father was present to sign the sale deed in 2009. On presenting the death certificate, their lawyer has stated that if we claim the said sale deed (a government document) done in 2009 is forged then the death certificate (also a government document) is also forged. Our suit is for cancellation of the said 2009 sale deed as the property owner (my father) died in 1998 and hence it is understood that the defendant presented unidentified person during sale deed registration in 2009. Can anyone claim that the death certificate issued by Registrar of Births & Deaths is forged.