Hi
1) Though you were designated as a nominee in your mother's AP Family benefit fund employee's contribution, your legal status is that of being a guardian to the said funds lying in the account post your mother's demise.
2) As a nominee, a duty was cast on you to distribute the funds in the said account to the legal heirs.
3) Having said so, you and your sister are the class I legal heirs of your mother and in absence of the will, both of you are entitled to equal share of the said funds.
4) Now your sister cannot claim any monies, rights under the guise of legal heir certificate as prima facie, legal heir certificate can only issued by courts and not by any other authority.
In defence of your claim you can rely on the following judgments(these judgments are applicable to your case given that your mother died in the year 1989)
a) You may also refer to judgment of he judgment of the Supreme Court in Faqruddin (Dead) through Lrs. v. Tajuddin (Dead) through Lrs. MANU/SC/2796/2008 : (2008) 8 SCC 12 wherein the Supreme Court held that revenue authorities cannot decide a question of title.
b) Another supreme court judgment in Suraj Bhan v. Financial Commissioner MANU/SC/7303/2007 : (2007) 6 SCC 186, wherein it was held that entries in the revenue records do not confer title and are only intended for achieving the fiscal purpose and no ownership is conferred on the basis of such entries. It was also held therein that so far as the title to the property is concerned, it could only be decided by a competent civil Court. The Supreme Court also held in Faqruddin's case (supra) that once the revenue authorities had no jurisdiction to decide the question of title, their decision being without jurisdiction would be a nullity.
c) Case in AP High court : K. Pratap Reddy and Ors. vs. The Joint Collector and Ors. (26.12.2008 - APHC) : MANU/AP/0704/2008 wherein the said directions of supreme court were followed.
Hope this information is useful.