1) file suit and seek injunction restraining step mother from selling properties standing in father name
2) on father demise your step mother , you and your siblings have equal share in property
3) if succession certificate has been fraudulently obtained apply to court for revocation of succession certificate
4)By virtue of Section 380 of the indian succession Act, a succession certificate granted under Part X of the Act shall have effect throughout India. The effect of issuing a Succession Certificate is that it will afford full indemnity to all persons owing the debts or liable on the securities specified in the certificate as regards all payments made or dealings had in good faith in respect of such debts or securities or with the grantee of the Succession Certificate. (Vide Section 381), A Succession Certificate issued under Part X of the Act is liable to be revoked for any of the following grounds:
a) that the proceedings to obtain the certificate were defective in substance.
b) that the certificate was obtained fraudulently by the making of a false suggestion, or by the concealment from the Court of something material to the case;
c) that the certificate was obtained by means of an untrue allegation of a fact essential in point of law to justify the grant thereof, though such allegation was made in ignorance or inadvertently;