First find out whether there are any other legal heirs.Next check for any encumbrances like mortgage etc.
If these two are the legal heirs then they are competent to enter into a sale transaction.For good order's sake publish a notice through a lawyer your intention to purchase the property and inviting any objections to the deal.
The sale deed drafted by a lawyer should trace the ownership of the property mentioning the relevant legal documents and as to how the present owners have a title to the property.Once all this is done get the sale deed registered.
In some states only Family Certificate is issued,meaning that Municipal authority or some such authority will merely give a certificate that so and so passed away and such and such are the family members.But this is not equivalent to Succession Certificate/Legal heirship certificate.It can be used for limited purposes.A propper SC/LHC can be given by a court application to which any one party has to make.Then after giving notice etc. and waiting for some period the court issues the certificate.This is a pucca document and nobody can challenge it.This is done mostly when the property is being distributed/partitioned and individual shares are to be identified.In such a case each legal heir gets a title to his/her share and can dispose off that share as deemed fit.
In your case since they are jointly disposing off the entire property and going to receive the consideration in lump sum,there is no such impediment.
Anyway,check with a lawyer.